cmd/goimports, imports: add -local flag

This allows the caller to indicate they want certain
import paths to sort into another group after 3rd-party
imports when added by goimports. For example, running
'goimports -local example.com/' might produce

    import (
        "database/sql"
        "io"
        "strconv"

        "golang.org/x/net/context"

        "example.com/foo/bar"
        "example.com/foo/baz"
    )

Resolves golang/go#12420

Change-Id: If6d88599f6cca2f102313bce95ba6ac46ffec1fe
Reviewed-on: https://go-review.googlesource.com/25145
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

cmd/goimports/goimports.go

@@ -49,6 +49,7 @@ var (
 
 func init() {
 	flag.BoolVar(&options.AllErrors, "e", false, "report all errors (not just the first 10 on different lines)")
+	flag.StringVar(&imports.LocalPrefix, "local", "", "put imports beginning with this string after 3rd-party packages")
 }
 
 func report(err error) {

imports/fix.go

@@ -32,9 +32,19 @@ var (
 	testMu  sync.RWMutex // guards globals reset by tests; used only if inTests
 )
 
+// If set, LocalPrefix instructs Process to sort import paths with the given
+// prefix into another group after 3rd-party packages.
+var LocalPrefix string
+
 // 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) {
+		if LocalPrefix != "" && strings.HasPrefix(importPath, LocalPrefix) {
+			return 3, true
+		}
+		return
+	},
 	func(importPath string) (num int, ok bool) {
 		if strings.HasPrefix(importPath, "appengine") {
 			return 2, true

imports/fix_test.go

@@ -1257,6 +1257,38 @@ const Y = bar.X
 	})
 }
 
+// Tests that the LocalPrefix option causes imports
+// to be added into a later group (num=3).
+func TestLocalPrefix(t *testing.T) {
+	defer func(s string) { LocalPrefix = s }(LocalPrefix)
+	LocalPrefix = "foo/"
+
+	testConfig{
+		gopathFiles: map[string]string{
+			"foo/bar/bar.go": "package bar \n const X = 1",
+		},
+	}.test(t, func(t *goimportTest) {
+		buf, err := Process(t.gopath+"/src/test/t.go", []byte("package main \n const Y = bar.X \n const _ = runtime.GOOS"), &Options{})
+		if err != nil {
+			t.Fatal(err)
+		}
+		const want = `package main
+
+import (
+	"runtime"
+
+	"foo/bar"
+)
+
+const Y = bar.X
+const _ = runtime.GOOS
+`
+		if string(buf) != want {
+			t.Errorf("Got:\n%s\nWant:\n%s", buf, want)
+		}
+	})
+}
+
 // Tests that running goimport on files in GOROOT (for people hacking
 // on Go itself) don't cause the GOPATH to be scanned (which might be
 // much bigger).