go.tools/imports: fix case where output not gofmt-compatible

Fixes 7866. Adds a test case.

When trying to add newlines before certain imports via text manipulation,
a regex is used to iterate over all imports. The regex failed to match
dot imports because \w doesn't match a literal dot. This changes the regex
to accept a dot as well.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/99400043

imports/fix_test.go

@@ -573,6 +573,49 @@ func main() {
 	//var _ = bar.A
 	var _ = qux.B
 }
+`,
+	},
+
+	// Blank line can be added before all types of import declarations.
+	// golang.org/issue/7866
+	{
+		name: "issue 7866",
+		in: `package main
+
+import (
+	"fmt"
+	renamed_bar "github.com/foo/bar"
+
+	. "github.com/foo/baz"
+	"io"
+
+	_ "github.com/foo/qux"
+	"strings"
+)
+
+func main() {
+	_, _, _, _, _ = fmt.Errorf, io.Copy, strings.Contains, renamed_bar.A, B
+}
+`,
+		out: `package main
+
+import (
+	"fmt"
+
+	renamed_bar "github.com/foo/bar"
+
+	"io"
+
+	. "github.com/foo/baz"
+
+	"strings"
+
+	_ "github.com/foo/qux"
+)
+
+func main() {
+	_, _, _, _, _ = fmt.Errorf, io.Copy, strings.Contains, renamed_bar.A, B
+}
 `,
 	},
 }

imports/imports.go

@@ -238,7 +238,7 @@ func matchSpace(orig []byte, src []byte) []byte {
 	return b.Bytes()
 }
 
-var impLine = regexp.MustCompile(`^\s+(?:\w+\s+)?"(.+)"`)
+var impLine = regexp.MustCompile(`^\s+(?:[\w\.]+\s+)?"(.+)"`)
 
 func addImportSpaces(r io.Reader, breaks []string) []byte {
 	var out bytes.Buffer