go/build: Return MultiplePackageError on importing a dir containing multiple packages

When the Import function in go/build encounters a directory
without any buildable Go source files, it returns a handy
NoGoError. Now if, instead it encounters multiple Go source files
from multiple packages, it returns a handy MultiplePackageError.

A new test for NoGoError and MultiplePackageError is also provided.

Fixes #8286.

LGTM=adg, rsc
R=bradfitz, rsc, adg
CC=golang-codereviews
https://golang.org/cl/155050043

src/go/build/build.go

@@ -417,6 +417,19 @@ func (e *NoGoError) Error() string {
 	return "no buildable Go source files in " + e.Dir
 }
 
+// MultiplePackageError describes a directory containing
+// multiple buildable Go source files for multiple packages.
+type MultiplePackageError struct {
+	Dir      string   // directory containing files
+	Packages []string // package names found
+	Files    []string // corresponding files: Files[i] declares package Packages[i]
+}
+
+func (e *MultiplePackageError) Error() string {
+	// Error string limited to two entries for compatibility.
+	return fmt.Sprintf("found packages %s (%s) and %s (%s) in %s", e.Packages[0], e.Files[0], e.Packages[1], e.Files[1], e.Dir)
+}
+
 func nameExt(name string) string {
 	i := strings.LastIndex(name, ".")
 	if i < 0 {
@@ -675,7 +688,7 @@ Found:
 			p.Name = pkg
 			firstFile = name
 		} else if pkg != p.Name {
-			return p, fmt.Errorf("found packages %s (%s) and %s (%s) in %s", p.Name, firstFile, pkg, name, p.Dir)
+			return p, &MultiplePackageError{p.Dir, []string{firstFile, name}, []string{p.Name, pkg}}
 		}
 		if pf.Doc != nil && p.Doc == "" {
 			p.Doc = doc.Synopsis(pf.Doc.Text())

src/go/build/build_test.go

@@ -85,6 +85,20 @@ func TestEmptyImport(t *testing.T) {
 	}
 }
 
+func TestEmptyFolderImport(t *testing.T) {
+	_, err := Import(".", "testdata/empty", 0)
+	if _, ok := err.(*NoGoError); !ok {
+		t.Fatal(`Import("testdata/empty") did not return NoGoError.`)
+	}
+}
+
+func TestMultiplePackageImport(t *testing.T) {
+	_, err := Import(".", "testdata/multi", 0)
+	if _, ok := err.(*MultiplePackageError); !ok {
+		t.Fatal(`Import("testdata/multi") did not return MultiplePackageError.`)
+	}
+}
+
 func TestLocalDirectory(t *testing.T) {
 	cwd, err := os.Getwd()
 	if err != nil {

src/go/build/testdata/multi/file.go

+// Test data - not compiled.
+
+package main
+
+func main() {}

src/go/build/testdata/multi/file_appengine.go

+// Test data - not compiled.
+
+package test_package
+
+func init() {}