imports: support match without trailing slash for -local flag

Make it so that import prefixes specified using the -local flag are considered
a match for an import path if the prefix ends with a '/' and the import path
matches exactly the prefix without a slash. For example, specifying
"golang.org/x/tools/" as a prefix would match the import for the package
"golang.org/x/tools".

Fixes golang/go#24368

Change-Id: I0302db72fda63ad24d7b964aa73f78aa0ebccb37
Reviewed-on: https://go-review.googlesource.com/100460
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

imports/fix.go

@@ -49,7 +49,7 @@ func localPrefixes() []string {
 var importToGroup = []func(importPath string) (num int, ok bool){
 	func(importPath string) (num int, ok bool) {
 		for _, p := range localPrefixes() {
-			if strings.HasPrefix(importPath, p) {
+			if strings.HasPrefix(importPath, p) || strings.TrimSuffix(p, "/") == importPath {
 				return 3, true
 			}
 		}

imports/fix_test.go

@@ -1444,6 +1444,29 @@ import (
 
 const Y = bar.X
 const _ = runtime.GOOS
+`,
+		},
+		{
+			config: testConfig{
+				gopathFiles: map[string]string{
+					"foo/foo.go":     "package foo \n const X = 1",
+					"foo/bar/bar.go": "package bar \n const X = 1",
+				},
+			},
+			localPrefix: "foo/",
+			src:         "package main \n const Y = bar.X \n const Z = foo.X \n const _ = runtime.GOOS",
+			want: `package main
+
+import (
+	"runtime"
+
+	"foo"
+	"foo/bar"
+)
+
+const Y = bar.X
+const Z = foo.X
+const _ = runtime.GOOS
 `,
 		},
 		{