diff --git a/go/packages/packagestest/modules.go b/go/packages/packagestest/modules.go
index 0a660bb1..2e06612c 100644
--- a/go/packages/packagestest/modules.go
+++ b/go/packages/packagestest/modules.go
@@ -56,7 +56,13 @@ func (modules) Finalize(exported *Exported) error {
 	// other weird stuff, and will be the working dir for the go command.
 	// It depends on all the other modules.
 	primaryDir := primaryDir(exported)
+	if err := os.MkdirAll(primaryDir, 0755); err != nil {
+		return err
+	}
 	exported.Config.Dir = primaryDir
+	if exported.written[exported.primary] == nil {
+		exported.written[exported.primary] = make(map[string]string)
+	}
 	exported.written[exported.primary]["go.mod"] = filepath.Join(primaryDir, "go.mod")
 	primaryGomod := "module " + exported.primary + "\nrequire (\n"
 	for other := range exported.written {
diff --git a/imports/forward.go b/imports/forward.go
new file mode 100644
index 00000000..bb661d59
--- /dev/null
+++ b/imports/forward.go
@@ -0,0 +1,59 @@
+// Package imports implements a Go pretty-printer (like package "go/format")
+// that also adds or removes import statements as necessary.
+package imports // import "golang.org/x/tools/imports"
+
+import (
+	"go/build"
+
+	intimp "golang.org/x/tools/internal/imports"
+)
+
+// Options specifies options for processing files.
+type Options struct {
+	Fragment  bool // Accept fragment of a source file (no package statement)
+	AllErrors bool // Report all errors (not just the first 10 on different lines)
+
+	Comments  bool // Print comments (true if nil *Options provided)
+	TabIndent bool // Use tabs for indent (true if nil *Options provided)
+	TabWidth  int  // Tab width (8 if nil *Options provided)
+
+	FormatOnly bool // Disable the insertion and deletion of imports
+}
+
+// Debug controls verbose logging.
+var Debug = false
+
+// LocalPrefix is a comma-separated string of import path prefixes, which, if
+// set, instructs Process to sort the import paths with the given prefixes
+// into another group after 3rd-party packages.
+var LocalPrefix string
+
+// Process formats and adjusts imports for the provided file.
+// If opt is nil the defaults are used.
+//
+// Note that filename's directory influences which imports can be chosen,
+// so it is important that filename be accurate.
+// To process data ``as if'' it were in filename, pass the data as a non-nil src.
+func Process(filename string, src []byte, opt *Options) ([]byte, error) {
+	intopt := &intimp.Options{
+		Env: &intimp.ProcessEnv{
+			GOPATH:      build.Default.GOPATH,
+			GOROOT:      build.Default.GOROOT,
+			Debug:       Debug,
+			LocalPrefix: LocalPrefix,
+		},
+		AllErrors:  opt.AllErrors,
+		Comments:   opt.Comments,
+		FormatOnly: opt.FormatOnly,
+		Fragment:   opt.Fragment,
+		TabIndent:  opt.TabIndent,
+		TabWidth:   opt.TabWidth,
+	}
+	return intimp.Process(filename, src, intopt)
+}
+
+// VendorlessPath returns the devendorized version of the import path ipath.
+// For example, VendorlessPath("foo/bar/vendor/a/b") returns "a/b".
+func VendorlessPath(ipath string) string {
+	return intimp.VendorlessPath(ipath)
+}
diff --git a/imports/fix.go b/internal/imports/fix.go
similarity index 94%
rename from imports/fix.go
rename to internal/imports/fix.go
index 777d28cc..3af3a142 100644
--- a/imports/fix.go
+++ b/internal/imports/fix.go
@@ -31,39 +31,27 @@ import (
 	"golang.org/x/tools/internal/gopathwalk"
 )
 
-// Debug controls verbose logging.
-var Debug = false
-
-// LocalPrefix is a comma-separated string of import path prefixes, which, if
-// set, instructs Process to sort the import paths with the given prefixes
-// into another group after 3rd-party packages.
-var LocalPrefix string
-
-func localPrefixes() []string {
-	if LocalPrefix != "" {
-		return strings.Split(LocalPrefix, ",")
-	}
-	return nil
-}
-
 // importToGroup is a list of functions which map from an import path to
 // a group number.
-var importToGroup = []func(importPath string) (num int, ok bool){
-	func(importPath string) (num int, ok bool) {
-		for _, p := range localPrefixes() {
+var importToGroup = []func(env *ProcessEnv, importPath string) (num int, ok bool){
+	func(env *ProcessEnv, importPath string) (num int, ok bool) {
+		if env.LocalPrefix == "" {
+			return
+		}
+		for _, p := range strings.Split(env.LocalPrefix, ",") {
 			if strings.HasPrefix(importPath, p) || strings.TrimSuffix(p, "/") == importPath {
 				return 3, true
 			}
 		}
 		return
 	},
-	func(importPath string) (num int, ok bool) {
+	func(_ *ProcessEnv, importPath string) (num int, ok bool) {
 		if strings.HasPrefix(importPath, "appengine") {
 			return 2, true
 		}
 		return
 	},
-	func(importPath string) (num int, ok bool) {
+	func(_ *ProcessEnv, importPath string) (num int, ok bool) {
 		if strings.Contains(importPath, ".") {
 			return 1, true
 		}
@@ -71,9 +59,9 @@ var importToGroup = []func(importPath string) (num int, ok bool){
 	},
 }
 
-func importGroup(importPath string) int {
+func importGroup(env *ProcessEnv, importPath string) int {
 	for _, fn := range importToGroup {
-		if n, ok := fn(importPath); ok {
+		if n, ok := fn(env, importPath); ok {
 			return n
 		}
 	}
@@ -241,7 +229,7 @@ type pass struct {
 	fset                 *token.FileSet // fset used to parse f and its siblings.
 	f                    *ast.File      // the file being fixed.
 	srcDir               string         // the directory containing f.
-	fixEnv               *fixEnv        // the environment to use for go commands, etc.
+	env                  *ProcessEnv    // the environment to use for go commands, etc.
 	loadRealPackageNames bool           // if true, load package names from disk rather than guessing them.
 	otherFiles           []*ast.File    // sibling files.
 
@@ -266,7 +254,7 @@ func (p *pass) loadPackageNames(imports []*importInfo) error {
 		unknown = append(unknown, imp.importPath)
 	}
 
-	names, err := p.fixEnv.getResolver().loadPackageNames(unknown, p.srcDir)
+	names, err := p.env.getResolver().loadPackageNames(unknown, p.srcDir)
 	if err != nil {
 		return err
 	}
@@ -324,7 +312,7 @@ func (p *pass) load() bool {
 	if p.loadRealPackageNames {
 		err := p.loadPackageNames(append(imports, p.candidates...))
 		if err != nil {
-			if Debug {
+			if p.env.Debug {
 				log.Printf("loading package names: %v", err)
 			}
 			return false
@@ -448,13 +436,13 @@ func (p *pass) addCandidate(imp *importInfo, pkg *packageInfo) {
 // easily be extended by adding a file with an init function.
 var fixImports = fixImportsDefault
 
-func fixImportsDefault(fset *token.FileSet, f *ast.File, filename string, env *fixEnv) error {
+func fixImportsDefault(fset *token.FileSet, f *ast.File, filename string, env *ProcessEnv) error {
 	abs, err := filepath.Abs(filename)
 	if err != nil {
 		return err
 	}
 	srcDir := filepath.Dir(abs)
-	if Debug {
+	if env.Debug {
 		log.Printf("fixImports(filename=%q), abs=%q, srcDir=%q ...", filename, abs, srcDir)
 	}
 
@@ -486,7 +474,7 @@ func fixImportsDefault(fset *token.FileSet, f *ast.File, filename string, env *f
 	// Third pass: get real package names where we had previously used
 	// the naive algorithm. This is the first step that will use the
 	// environment, so we provide it here for the first time.
-	p = &pass{fset: fset, f: f, srcDir: srcDir, fixEnv: env}
+	p = &pass{fset: fset, f: f, srcDir: srcDir, env: env}
 	p.loadRealPackageNames = true
 	p.otherFiles = otherFiles
 	if p.load() {
@@ -510,9 +498,12 @@ func fixImportsDefault(fset *token.FileSet, f *ast.File, filename string, env *f
 	return nil
 }
 
-// fixEnv contains environment variables and settings that affect the use of
+// ProcessEnv contains environment variables and settings that affect the use of
 // the go command, the go/build package, etc.
-type fixEnv struct {
+type ProcessEnv struct {
+	LocalPrefix string
+	Debug       bool
+
 	// If non-empty, these will be used instead of the
 	// process-wide values.
 	GOPATH, GOROOT, GO111MODULE, GOPROXY, GOFLAGS string
@@ -524,7 +515,7 @@ type fixEnv struct {
 	resolver resolver
 }
 
-func (e *fixEnv) env() []string {
+func (e *ProcessEnv) env() []string {
 	env := os.Environ()
 	add := func(k, v string) {
 		if v != "" {
@@ -542,7 +533,7 @@ func (e *fixEnv) env() []string {
 	return env
 }
 
-func (e *fixEnv) getResolver() resolver {
+func (e *ProcessEnv) getResolver() resolver {
 	if e.resolver != nil {
 		return e.resolver
 	}
@@ -557,7 +548,7 @@ func (e *fixEnv) getResolver() resolver {
 	return &moduleResolver{env: e}
 }
 
-func (e *fixEnv) newPackagesConfig(mode packages.LoadMode) *packages.Config {
+func (e *ProcessEnv) newPackagesConfig(mode packages.LoadMode) *packages.Config {
 	return &packages.Config{
 		Mode: mode,
 		Dir:  e.WorkingDir,
@@ -565,14 +556,14 @@ func (e *fixEnv) newPackagesConfig(mode packages.LoadMode) *packages.Config {
 	}
 }
 
-func (e *fixEnv) buildContext() *build.Context {
+func (e *ProcessEnv) buildContext() *build.Context {
 	ctx := build.Default
 	ctx.GOROOT = e.GOROOT
 	ctx.GOPATH = e.GOPATH
 	return &ctx
 }
 
-func (e *fixEnv) invokeGo(args ...string) (*bytes.Buffer, error) {
+func (e *ProcessEnv) invokeGo(args ...string) (*bytes.Buffer, error) {
 	cmd := exec.Command("go", args...)
 	stdout := &bytes.Buffer{}
 	stderr := &bytes.Buffer{}
@@ -581,7 +572,7 @@ func (e *fixEnv) invokeGo(args ...string) (*bytes.Buffer, error) {
 	cmd.Env = e.env()
 	cmd.Dir = e.WorkingDir
 
-	if Debug {
+	if e.Debug {
 		defer func(start time.Time) { log.Printf("%s for %v", time.Since(start), cmdDebugStr(cmd)) }(time.Now())
 	}
 	if err := cmd.Run(); err != nil {
@@ -632,7 +623,7 @@ type resolver interface {
 
 // gopathResolver implements resolver for GOPATH and module workspaces using go/packages.
 type goPackagesResolver struct {
-	env *fixEnv
+	env *ProcessEnv
 }
 
 func (r *goPackagesResolver) loadPackageNames(importPaths []string, srcDir string) (map[string]string, error) {
@@ -680,7 +671,7 @@ func (r *goPackagesResolver) scan(refs references) ([]*pkg, error) {
 }
 
 func addExternalCandidates(pass *pass, refs references, filename string) error {
-	dirScan, err := pass.fixEnv.getResolver().scan(refs)
+	dirScan, err := pass.env.getResolver().scan(refs)
 	if err != nil {
 		return err
 	}
@@ -707,7 +698,7 @@ func addExternalCandidates(pass *pass, refs references, filename string) error {
 		go func(pkgName string, symbols map[string]bool) {
 			defer wg.Done()
 
-			found, err := findImport(ctx, pass.fixEnv, dirScan, pkgName, symbols, filename)
+			found, err := findImport(ctx, pass.env, dirScan, pkgName, symbols, filename)
 
 			if err != nil {
 				firstErrOnce.Do(func() {
@@ -778,7 +769,7 @@ func importPathToAssumedName(importPath string) string {
 
 // gopathResolver implements resolver for GOPATH workspaces.
 type gopathResolver struct {
-	env *fixEnv
+	env *ProcessEnv
 }
 
 func (r *gopathResolver) loadPackageNames(importPaths []string, srcDir string) (map[string]string, error) {
@@ -791,7 +782,7 @@ func (r *gopathResolver) loadPackageNames(importPaths []string, srcDir string) (
 
 // importPathToNameGoPath finds out the actual package name, as declared in its .go files.
 // If there's a problem, it returns "".
-func importPathToName(env *fixEnv, importPath, srcDir string) (packageName string) {
+func importPathToName(env *ProcessEnv, importPath, srcDir string) (packageName string) {
 	// Fast path for standard library without going to disk.
 	if _, ok := stdlib[importPath]; ok {
 		return path.Base(importPath) // stdlib packages always match their paths.
@@ -927,7 +918,7 @@ func (r *gopathResolver) scan(_ references) ([]*pkg, error) {
 			dir:             dir,
 		})
 	}
-	gopathwalk.Walk(gopathwalk.SrcDirsRoots(r.env.buildContext()), add, gopathwalk.Options{Debug: Debug, ModulesEnabled: false})
+	gopathwalk.Walk(gopathwalk.SrcDirsRoots(r.env.buildContext()), add, gopathwalk.Options{Debug: r.env.Debug, ModulesEnabled: false})
 	return result, nil
 }
 
@@ -946,8 +937,8 @@ func VendorlessPath(ipath string) string {
 
 // loadExports returns the set of exported symbols in the package at dir.
 // It returns nil on error or if the package name in dir does not match expectPackage.
-func loadExports(ctx context.Context, env *fixEnv, expectPackage string, pkg *pkg) (map[string]bool, error) {
-	if Debug {
+func loadExports(ctx context.Context, env *ProcessEnv, expectPackage string, pkg *pkg) (map[string]bool, error) {
+	if env.Debug {
 		log.Printf("loading exports in dir %s (seeking package %s)", pkg.dir, expectPackage)
 	}
 	if pkg.goPackage != nil {
@@ -1020,7 +1011,7 @@ func loadExports(ctx context.Context, env *fixEnv, expectPackage string, pkg *pk
 		}
 	}
 
-	if Debug {
+	if env.Debug {
 		exportList := make([]string, 0, len(exports))
 		for k := range exports {
 			exportList = append(exportList, k)
@@ -1033,7 +1024,7 @@ func loadExports(ctx context.Context, env *fixEnv, expectPackage string, pkg *pk
 
 // findImport searches for a package with the given symbols.
 // If no package is found, findImport returns ("", false, nil)
-func findImport(ctx context.Context, env *fixEnv, dirScan []*pkg, pkgName string, symbols map[string]bool, filename string) (*pkg, error) {
+func findImport(ctx context.Context, env *ProcessEnv, dirScan []*pkg, pkgName string, symbols map[string]bool, filename string) (*pkg, error) {
 	pkgDir, err := filepath.Abs(filename)
 	if err != nil {
 		return nil, err
@@ -1056,7 +1047,7 @@ func findImport(ctx context.Context, env *fixEnv, dirScan []*pkg, pkgName string
 	// ones.  Note that this sorts by the de-vendored name, so
 	// there's no "penalty" for vendoring.
 	sort.Sort(byDistanceOrImportPathShortLength(candidates))
-	if Debug {
+	if env.Debug {
 		for i, c := range candidates {
 			log.Printf("%s candidate %d/%d: %v in %v", pkgName, i+1, len(candidates), c.pkg.importPathShort, c.pkg.dir)
 		}
@@ -1097,7 +1088,7 @@ func findImport(ctx context.Context, env *fixEnv, dirScan []*pkg, pkgName string
 
 				exports, err := loadExports(ctx, env, pkgName, c.pkg)
 				if err != nil {
-					if Debug {
+					if env.Debug {
 						log.Printf("loading exports in dir %s (seeking package %s): %v", c.pkg.dir, pkgName, err)
 					}
 					resc <- nil
diff --git a/imports/fix_test.go b/internal/imports/fix_test.go
similarity index 94%
rename from imports/fix_test.go
rename to internal/imports/fix_test.go
index 101978c1..4f106969 100644
--- a/imports/fix_test.go
+++ b/internal/imports/fix_test.go
@@ -5,6 +5,7 @@
 package imports
 
 import (
+	"flag"
 	"fmt"
 	"path/filepath"
 	"runtime"
@@ -14,6 +15,8 @@ import (
 	"golang.org/x/tools/go/packages/packagestest"
 )
 
+var testDebug = flag.Bool("debug", false, "enable debug output")
+
 var tests = []struct {
 	name       string
 	formatOnly bool
@@ -1116,8 +1119,7 @@ var _, _ = rand.Read, rand.NewZipf
 }
 
 func TestSimpleCases(t *testing.T) {
-	defer func(lp string) { LocalPrefix = lp }(LocalPrefix)
-	LocalPrefix = "local.com,github.com/local"
+	const localPrefix = "local.com,github.com/local"
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			options := &Options{
@@ -1163,7 +1165,11 @@ func TestSimpleCases(t *testing.T) {
 						Files: fm{"bar/x.go": "package bar\nfunc Bar(){}\n"},
 					},
 				},
-			}.processTest(t, "golang.org/fake", "x.go", nil, options, tt.out)
+			}.test(t, func(t *goimportTest) {
+				t.env.LocalPrefix = localPrefix
+				t.assertProcessEquals("golang.org/fake", "x.go", nil, options, tt.out)
+			})
+
 		})
 	}
 }
@@ -1485,20 +1491,28 @@ func TestFindStdlib(t *testing.T) {
 		for _, sym := range tt.symbols {
 			input += fmt.Sprintf("var _ = %s.%s\n", tt.pkg, sym)
 		}
-		buf, err := Process("x.go", []byte(input), &Options{})
-		if err != nil {
-			t.Fatal(err)
-		}
-		if got := string(buf); !strings.Contains(got, tt.want) {
-			t.Errorf("Process(%q) = %q, wanted it to contain %q", input, buf, tt.want)
-		}
+		testConfig{
+			module: packagestest.Module{
+				Name:  "foo.com",
+				Files: fm{"x.go": input},
+			},
+		}.test(t, func(t *goimportTest) {
+			buf, err := t.process("foo.com", "x.go", nil, nil)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if got := string(buf); !strings.Contains(got, tt.want) {
+				t.Errorf("Process(%q) = %q, wanted it to contain %q", input, buf, tt.want)
+			}
+		})
 	}
 }
 
 type testConfig struct {
-	gopathOnly bool
-	module     packagestest.Module
-	modules    []packagestest.Module
+	gopathOnly             bool
+	goPackagesIncompatible bool
+	module                 packagestest.Module
+	modules                []packagestest.Module
 }
 
 // fm is the type for a packagestest.Module's Files, abbreviated for shorter lines.
@@ -1522,6 +1536,12 @@ func (c testConfig) test(t *testing.T, fn func(*goimportTest)) {
 
 			forceGoPackages := false
 			var exporter packagestest.Exporter
+			if c.gopathOnly && strings.HasPrefix(kind, "Modules") {
+				t.Skip("test marked GOPATH-only")
+			}
+			if c.goPackagesIncompatible && strings.HasSuffix(kind, "_GoPackages") {
+				t.Skip("test marked go/packages-incompatible")
+			}
 			switch kind {
 			case "GOPATH":
 				exporter = packagestest.GOPATH
@@ -1529,14 +1549,8 @@ func (c testConfig) test(t *testing.T, fn func(*goimportTest)) {
 				exporter = packagestest.GOPATH
 				forceGoPackages = true
 			case "Modules":
-				if c.gopathOnly {
-					t.Skip("test marked GOPATH-only")
-				}
 				exporter = packagestest.Modules
 			case "Modules_GoPackages":
-				if c.gopathOnly {
-					t.Skip("test marked GOPATH-only")
-				}
 				exporter = packagestest.Modules
 				forceGoPackages = true
 			default:
@@ -1554,12 +1568,13 @@ func (c testConfig) test(t *testing.T, fn func(*goimportTest)) {
 
 			it := &goimportTest{
 				T: t,
-				fixEnv: &fixEnv{
+				env: &ProcessEnv{
 					GOROOT:          env["GOROOT"],
 					GOPATH:          env["GOPATH"],
 					GO111MODULE:     env["GO111MODULE"],
 					WorkingDir:      exported.Config.Dir,
 					ForceGoPackages: forceGoPackages,
+					Debug:           *testDebug,
 				},
 				exported: exported,
 			}
@@ -1572,23 +1587,36 @@ func (c testConfig) processTest(t *testing.T, module, file string, contents []by
 	t.Helper()
 	c.test(t, func(t *goimportTest) {
 		t.Helper()
-		t.process(module, file, contents, opts, want)
+		t.assertProcessEquals(module, file, contents, opts, want)
 	})
 }
 
 type goimportTest struct {
 	*testing.T
-	fixEnv   *fixEnv
+	env      *ProcessEnv
 	exported *packagestest.Exported
 }
 
-func (t *goimportTest) process(module, file string, contents []byte, opts *Options, want string) {
+func (t *goimportTest) process(module, file string, contents []byte, opts *Options) ([]byte, error) {
 	t.Helper()
 	f := t.exported.File(module, file)
 	if f == "" {
 		t.Fatalf("%v not found in exported files (typo in filename?)", file)
 	}
-	buf, err := process(f, contents, opts, t.fixEnv)
+	return t.processNonModule(f, contents, opts)
+}
+
+func (t *goimportTest) processNonModule(file string, contents []byte, opts *Options) ([]byte, error) {
+	if opts == nil {
+		opts = &Options{Comments: true, TabIndent: true, TabWidth: 8}
+	}
+	opts.Env = t.env
+	opts.Env.Debug = *testDebug
+	return Process(file, contents, opts)
+}
+
+func (t *goimportTest) assertProcessEquals(module, file string, contents []byte, opts *Options, want string) {
+	buf, err := t.process(module, file, contents, opts)
 	if err != nil {
 		t.Fatalf("Process() = %v", err)
 	}
@@ -1775,9 +1803,8 @@ const _ = runtime.GOOS
 					Files: fm{"t.go": tt.src},
 				}}, tt.modules...),
 			}.test(t, func(t *goimportTest) {
-				defer func(s string) { LocalPrefix = s }(LocalPrefix)
-				LocalPrefix = tt.localPrefix
-				t.process("test.com", "t.go", nil, nil, tt.want)
+				t.env.LocalPrefix = tt.localPrefix
+				t.assertProcessEquals("test.com", "t.go", nil, nil, tt.want)
 			})
 		})
 	}
@@ -1827,7 +1854,7 @@ func TestImportPathToNameGoPathParse(t *testing.T) {
 		if strings.Contains(t.Name(), "GoPackages") {
 			t.Skip("go/packages does not ignore package main")
 		}
-		r := t.fixEnv.getResolver()
+		r := t.env.getResolver()
 		srcDir := filepath.Dir(t.exported.File("example.net/pkg", "z.go"))
 		names, err := r.loadPackageNames([]string{"example.net/pkg"}, srcDir)
 		if err != nil {
@@ -2220,13 +2247,19 @@ func TestPkgIsCandidate(t *testing.T) {
 
 // Issue 20941: this used to panic on Windows.
 func TestProcessStdin(t *testing.T) {
-	got, err := Process("<standard input>", []byte("package main\nfunc main() {\n\tfmt.Println(123)\n}\n"), nil)
-	if err != nil {
-		t.Fatal(err)
-	}
-	if !strings.Contains(string(got), `"fmt"`) {
-		t.Errorf("expected fmt import; got: %s", got)
-	}
+	testConfig{
+		module: packagestest.Module{
+			Name: "foo.com",
+		},
+	}.test(t, func(t *goimportTest) {
+		got, err := t.processNonModule("<standard input>", []byte("package main\nfunc main() {\n\tfmt.Println(123)\n}\n"), nil)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !strings.Contains(string(got), `"fmt"`) {
+			t.Errorf("expected fmt import; got: %s", got)
+		}
+	})
 }
 
 // Tests LocalPackagePromotion when there is a local package that matches, it
@@ -2324,14 +2357,21 @@ import "bytes"
 
 var _ = &bytes.Buffer{}
 `
+	testConfig{
+		goPackagesIncompatible: true,
+		module: packagestest.Module{
+			Name: "mycompany.net",
+		},
+	}.test(t, func(t *goimportTest) {
+		buf, err := t.processNonModule("mycompany.net/tool/main.go", []byte(input), nil)
+		if err != nil {
+			t.Fatalf("Process() = %v", err)
+		}
+		if string(buf) != want {
+			t.Errorf("Got:\n%s\nWant:\n%s", buf, want)
+		}
+	})
 
-	buf, err := Process("mycompany.net/tool/main.go", []byte(input), nil)
-	if err != nil {
-		t.Fatalf("Process() = %v", err)
-	}
-	if string(buf) != want {
-		t.Errorf("Got:\n%s\nWant:\n%s", buf, want)
-	}
 }
 
 // Ensures a token as large as 500000 bytes can be handled
diff --git a/imports/imports.go b/internal/imports/imports.go
similarity index 91%
rename from imports/imports.go
rename to internal/imports/imports.go
index 07101cb8..8b63908c 100644
--- a/imports/imports.go
+++ b/internal/imports/imports.go
@@ -6,14 +6,13 @@
 
 // Package imports implements a Go pretty-printer (like package "go/format")
 // that also adds or removes import statements as necessary.
-package imports // import "golang.org/x/tools/imports"
+package imports
 
 import (
 	"bufio"
 	"bytes"
 	"fmt"
 	"go/ast"
-	"go/build"
 	"go/format"
 	"go/parser"
 	"go/printer"
@@ -27,8 +26,10 @@ import (
 	"golang.org/x/tools/go/ast/astutil"
 )
 
-// Options specifies options for processing files.
+// Options is golang.org/x/tools/imports.Options with extra internal-only options.
 type Options struct {
+	Env *ProcessEnv // The environment to use. Note: this contains the cached module and filesystem state.
+
 	Fragment  bool // Accept fragment of a source file (no package statement)
 	AllErrors bool // Report all errors (not just the first 10 on different lines)
 
@@ -39,18 +40,8 @@ type Options struct {
 	FormatOnly bool // Disable the insertion and deletion of imports
 }
 
-// Process formats and adjusts imports for the provided file.
-// If opt is nil the defaults are used.
-//
-// Note that filename's directory influences which imports can be chosen,
-// so it is important that filename be accurate.
-// To process data ``as if'' it were in filename, pass the data as a non-nil src.
+// Process implements golang.org/x/tools/imports.Process with explicit context in env.
 func Process(filename string, src []byte, opt *Options) ([]byte, error) {
-	env := &fixEnv{GOPATH: build.Default.GOPATH, GOROOT: build.Default.GOROOT}
-	return process(filename, src, opt, env)
-}
-
-func process(filename string, src []byte, opt *Options, env *fixEnv) ([]byte, error) {
 	if opt == nil {
 		opt = &Options{Comments: true, TabIndent: true, TabWidth: 8}
 	}
@@ -69,12 +60,12 @@ func process(filename string, src []byte, opt *Options, env *fixEnv) ([]byte, er
 	}
 
 	if !opt.FormatOnly {
-		if err := fixImports(fileSet, file, filename, env); err != nil {
+		if err := fixImports(fileSet, file, filename, opt.Env); err != nil {
 			return nil, err
 		}
 	}
 
-	sortImports(fileSet, file)
+	sortImports(opt.Env, fileSet, file)
 	imps := astutil.Imports(fileSet, file)
 	var spacesBefore []string // import paths we need spaces before
 	for _, impSection := range imps {
@@ -85,7 +76,7 @@ func process(filename string, src []byte, opt *Options, env *fixEnv) ([]byte, er
 		lastGroup := -1
 		for _, importSpec := range impSection {
 			importPath, _ := strconv.Unquote(importSpec.Path.Value)
-			groupNum := importGroup(importPath)
+			groupNum := importGroup(opt.Env, importPath)
 			if groupNum != lastGroup && lastGroup != -1 {
 				spacesBefore = append(spacesBefore, importPath)
 			}
diff --git a/imports/mkindex.go b/internal/imports/mkindex.go
similarity index 99%
rename from imports/mkindex.go
rename to internal/imports/mkindex.go
index 755e2394..ef8c0d28 100644
--- a/imports/mkindex.go
+++ b/internal/imports/mkindex.go
@@ -8,7 +8,7 @@
 // standard library. The file is intended to be built as part of the imports
 // package, so that the package may be used in environments where a GOROOT is
 // not available (such as App Engine).
-package main
+package imports
 
 import (
 	"bytes"
diff --git a/imports/mkstdlib.go b/internal/imports/mkstdlib.go
similarity index 99%
rename from imports/mkstdlib.go
rename to internal/imports/mkstdlib.go
index c8865e55..f67b5c1e 100644
--- a/imports/mkstdlib.go
+++ b/internal/imports/mkstdlib.go
@@ -3,7 +3,7 @@
 // mkstdlib generates the zstdlib.go file, containing the Go standard
 // library API symbols. It's baked into the binary to avoid scanning
 // GOPATH in the common case.
-package main
+package imports
 
 import (
 	"bufio"
diff --git a/imports/mod.go b/internal/imports/mod.go
similarity index 98%
rename from imports/mod.go
rename to internal/imports/mod.go
index 018c43ce..a072214e 100644
--- a/imports/mod.go
+++ b/internal/imports/mod.go
@@ -22,7 +22,7 @@ import (
 // moduleResolver implements resolver for modules using the go command as little
 // as feasible.
 type moduleResolver struct {
-	env *fixEnv
+	env *ProcessEnv
 
 	initialized   bool
 	main          *moduleJSON
@@ -62,7 +62,7 @@ func (r *moduleResolver) init() error {
 			return err
 		}
 		if mod.Dir == "" {
-			if Debug {
+			if r.env.Debug {
 				log.Printf("module %v has not been downloaded and will be ignored", mod.Path)
 			}
 			// Can't do anything with a module that's not downloaded.
@@ -253,7 +253,7 @@ func (r *moduleResolver) scan(_ references) ([]*pkg, error) {
 			matches := modCacheRegexp.FindStringSubmatch(subdir)
 			modPath, err := module.DecodePath(filepath.ToSlash(matches[1]))
 			if err != nil {
-				if Debug {
+				if r.env.Debug {
 					log.Printf("decoding module cache path %q: %v", subdir, err)
 				}
 				return
@@ -303,7 +303,7 @@ func (r *moduleResolver) scan(_ references) ([]*pkg, error) {
 			importPathShort: VendorlessPath(importPath),
 			dir:             dir,
 		})
-	}, gopathwalk.Options{Debug: Debug, ModulesEnabled: true})
+	}, gopathwalk.Options{Debug: r.env.Debug, ModulesEnabled: true})
 	return result, nil
 }
 
diff --git a/imports/mod_112_test.go b/internal/imports/mod_112_test.go
similarity index 100%
rename from imports/mod_112_test.go
rename to internal/imports/mod_112_test.go
diff --git a/imports/mod_test.go b/internal/imports/mod_test.go
similarity index 99%
rename from imports/mod_test.go
rename to internal/imports/mod_test.go
index f2fb76e2..cda41470 100644
--- a/imports/mod_test.go
+++ b/internal/imports/mod_test.go
@@ -485,7 +485,7 @@ var proxyDir string
 
 type modTest struct {
 	*testing.T
-	env      *fixEnv
+	env      *ProcessEnv
 	resolver *moduleResolver
 	cleanup  func()
 }
@@ -515,7 +515,7 @@ func setup(t *testing.T, main, wd string) *modTest {
 		t.Fatal(err)
 	}
 
-	env := &fixEnv{
+	env := &ProcessEnv{
 		GOROOT:      build.Default.GOROOT,
 		GOPATH:      filepath.Join(dir, "gopath"),
 		GO111MODULE: "on",
diff --git a/imports/proxy_112_test.go b/internal/imports/proxy_112_test.go
similarity index 100%
rename from imports/proxy_112_test.go
rename to internal/imports/proxy_112_test.go
diff --git a/imports/proxy_113_test.go b/internal/imports/proxy_113_test.go
similarity index 100%
rename from imports/proxy_113_test.go
rename to internal/imports/proxy_113_test.go
diff --git a/imports/sortimports.go b/internal/imports/sortimports.go
similarity index 85%
rename from imports/sortimports.go
rename to internal/imports/sortimports.go
index f3dd56c7..0a156fe2 100644
--- a/imports/sortimports.go
+++ b/internal/imports/sortimports.go
@@ -15,7 +15,7 @@ import (
 
 // sortImports sorts runs of consecutive import lines in import blocks in f.
 // It also removes duplicate imports when it is possible to do so without data loss.
-func sortImports(fset *token.FileSet, f *ast.File) {
+func sortImports(env *ProcessEnv, fset *token.FileSet, f *ast.File) {
 	for i, d := range f.Decls {
 		d, ok := d.(*ast.GenDecl)
 		if !ok || d.Tok != token.IMPORT {
@@ -40,11 +40,11 @@ func sortImports(fset *token.FileSet, f *ast.File) {
 		for j, s := range d.Specs {
 			if j > i && fset.Position(s.Pos()).Line > 1+fset.Position(d.Specs[j-1].End()).Line {
 				// j begins a new run.  End this one.
-				specs = append(specs, sortSpecs(fset, f, d.Specs[i:j])...)
+				specs = append(specs, sortSpecs(env, fset, f, d.Specs[i:j])...)
 				i = j
 			}
 		}
-		specs = append(specs, sortSpecs(fset, f, d.Specs[i:])...)
+		specs = append(specs, sortSpecs(env, fset, f, d.Specs[i:])...)
 		d.Specs = specs
 
 		// Deduping can leave a blank line before the rparen; clean that up.
@@ -95,7 +95,7 @@ type posSpan struct {
 	End   token.Pos
 }
 
-func sortSpecs(fset *token.FileSet, f *ast.File, specs []ast.Spec) []ast.Spec {
+func sortSpecs(env *ProcessEnv, fset *token.FileSet, f *ast.File, specs []ast.Spec) []ast.Spec {
 	// Can't short-circuit here even if specs are already sorted,
 	// since they might yet need deduplication.
 	// A lone import, however, may be safely ignored.
@@ -144,7 +144,7 @@ func sortSpecs(fset *token.FileSet, f *ast.File, specs []ast.Spec) []ast.Spec {
 	// Reassign the import paths to have the same position sequence.
 	// Reassign each comment to abut the end of its spec.
 	// Sort the comments by new position.
-	sort.Sort(byImportSpec(specs))
+	sort.Sort(byImportSpec{env, specs})
 
 	// Dedup. Thanks to our sorting, we can just consider
 	// adjacent pairs of imports.
@@ -197,16 +197,19 @@ func sortSpecs(fset *token.FileSet, f *ast.File, specs []ast.Spec) []ast.Spec {
 	return specs
 }
 
-type byImportSpec []ast.Spec // slice of *ast.ImportSpec
+type byImportSpec struct {
+	env   *ProcessEnv
+	specs []ast.Spec // slice of *ast.ImportSpec
+}
 
-func (x byImportSpec) Len() int      { return len(x) }
-func (x byImportSpec) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
+func (x byImportSpec) Len() int      { return len(x.specs) }
+func (x byImportSpec) Swap(i, j int) { x.specs[i], x.specs[j] = x.specs[j], x.specs[i] }
 func (x byImportSpec) Less(i, j int) bool {
-	ipath := importPath(x[i])
-	jpath := importPath(x[j])
+	ipath := importPath(x.specs[i])
+	jpath := importPath(x.specs[j])
 
-	igroup := importGroup(ipath)
-	jgroup := importGroup(jpath)
+	igroup := importGroup(x.env, ipath)
+	jgroup := importGroup(x.env, jpath)
 	if igroup != jgroup {
 		return igroup < jgroup
 	}
@@ -214,13 +217,13 @@ func (x byImportSpec) Less(i, j int) bool {
 	if ipath != jpath {
 		return ipath < jpath
 	}
-	iname := importName(x[i])
-	jname := importName(x[j])
+	iname := importName(x.specs[i])
+	jname := importName(x.specs[j])
 
 	if iname != jname {
 		return iname < jname
 	}
-	return importComment(x[i]) < importComment(x[j])
+	return importComment(x.specs[i]) < importComment(x.specs[j])
 }
 
 type byCommentPos []*ast.CommentGroup
diff --git a/imports/testdata/mod/example.com_v1.0.0.txt b/internal/imports/testdata/mod/example.com_v1.0.0.txt
similarity index 100%
rename from imports/testdata/mod/example.com_v1.0.0.txt
rename to internal/imports/testdata/mod/example.com_v1.0.0.txt
diff --git a/imports/testdata/mod/golang.org_x_text_v0.0.0-20170915032832-14c0d48ead0c.txt b/internal/imports/testdata/mod/golang.org_x_text_v0.0.0-20170915032832-14c0d48ead0c.txt
similarity index 100%
rename from imports/testdata/mod/golang.org_x_text_v0.0.0-20170915032832-14c0d48ead0c.txt
rename to internal/imports/testdata/mod/golang.org_x_text_v0.0.0-20170915032832-14c0d48ead0c.txt
diff --git a/imports/testdata/mod/rsc.io_!q!u!o!t!e_v1.5.2.txt b/internal/imports/testdata/mod/rsc.io_!q!u!o!t!e_v1.5.2.txt
similarity index 100%
rename from imports/testdata/mod/rsc.io_!q!u!o!t!e_v1.5.2.txt
rename to internal/imports/testdata/mod/rsc.io_!q!u!o!t!e_v1.5.2.txt
diff --git a/imports/testdata/mod/rsc.io_!q!u!o!t!e_v1.5.3-!p!r!e.txt b/internal/imports/testdata/mod/rsc.io_!q!u!o!t!e_v1.5.3-!p!r!e.txt
similarity index 100%
rename from imports/testdata/mod/rsc.io_!q!u!o!t!e_v1.5.3-!p!r!e.txt
rename to internal/imports/testdata/mod/rsc.io_!q!u!o!t!e_v1.5.3-!p!r!e.txt
diff --git a/imports/testdata/mod/rsc.io_quote_v1.5.1.txt b/internal/imports/testdata/mod/rsc.io_quote_v1.5.1.txt
similarity index 100%
rename from imports/testdata/mod/rsc.io_quote_v1.5.1.txt
rename to internal/imports/testdata/mod/rsc.io_quote_v1.5.1.txt
diff --git a/imports/testdata/mod/rsc.io_quote_v1.5.2.txt b/internal/imports/testdata/mod/rsc.io_quote_v1.5.2.txt
similarity index 100%
rename from imports/testdata/mod/rsc.io_quote_v1.5.2.txt
rename to internal/imports/testdata/mod/rsc.io_quote_v1.5.2.txt
diff --git a/imports/testdata/mod/rsc.io_quote_v2_v2.0.1.txt b/internal/imports/testdata/mod/rsc.io_quote_v2_v2.0.1.txt
similarity index 100%
rename from imports/testdata/mod/rsc.io_quote_v2_v2.0.1.txt
rename to internal/imports/testdata/mod/rsc.io_quote_v2_v2.0.1.txt
diff --git a/imports/testdata/mod/rsc.io_quote_v3_v3.0.0.txt b/internal/imports/testdata/mod/rsc.io_quote_v3_v3.0.0.txt
similarity index 100%
rename from imports/testdata/mod/rsc.io_quote_v3_v3.0.0.txt
rename to internal/imports/testdata/mod/rsc.io_quote_v3_v3.0.0.txt
diff --git a/imports/testdata/mod/rsc.io_sampler_v1.3.0.txt b/internal/imports/testdata/mod/rsc.io_sampler_v1.3.0.txt
similarity index 100%
rename from imports/testdata/mod/rsc.io_sampler_v1.3.0.txt
rename to internal/imports/testdata/mod/rsc.io_sampler_v1.3.0.txt
diff --git a/imports/testdata/mod/rsc.io_sampler_v1.3.1.txt b/internal/imports/testdata/mod/rsc.io_sampler_v1.3.1.txt
similarity index 100%
rename from imports/testdata/mod/rsc.io_sampler_v1.3.1.txt
rename to internal/imports/testdata/mod/rsc.io_sampler_v1.3.1.txt
diff --git a/imports/zstdlib.go b/internal/imports/zstdlib.go
similarity index 100%
rename from imports/zstdlib.go
rename to internal/imports/zstdlib.go