cmd/go: include external test files in the files sent to gofmt, govet, and gofix

Also, add XTestGoFiles to the go command's public api.

Fixes #2649.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/5502102

src/cmd/go/list.go

@@ -36,8 +36,9 @@ being passed to the template is:
         Stale      bool   // would 'go install' do anything for this package?
 
         // Source files
-        GoFiles     []string // .go source files (excluding CgoFiles and TestGoFiles)
-        TestGoFiles []string // _test.go source files
+        GoFiles      []string // .go source files (excluding CgoFiles, TestGoFiles, and XTestGoFiles)
+        TestGoFiles  []string // _test.go source files internal to the package they are testing
+        XTestGoFiles []string // _test.go source files external to the package they are testing
         CFiles       []string // .c source files
         HFiles       []string // .h source files
         SFiles       []string // .s source files

src/cmd/go/pkg.go

@@ -29,8 +29,9 @@ type Package struct {
 	Stale      bool   `json:",omitempty"` // would 'go install' do anything for this package?
 
 	// Source files
-	GoFiles     []string // .go source files (excluding CgoFiles and TestGoFiles)
-	TestGoFiles []string `json:",omitempty"` // _test.go source files
+	GoFiles      []string // .go source files (excluding CgoFiles, TestGoFiles and XTestGoFiles)
+	TestGoFiles  []string `json:",omitempty"` // _test.go source files internal to the package they are testing
+	XTestGoFiles []string `json:",omitempty"` //_test.go source files external to the package they are testing
 	CFiles       []string `json:",omitempty"` // .c source files
 	HFiles       []string `json:",omitempty"` // .h source files
 	SFiles       []string `json:",omitempty"` // .s source files
@@ -47,7 +48,7 @@ type Package struct {
 	pkgdir  string
 	info    *build.DirInfo
 	imports []*Package
-	gofiles []string // GoFiles+CgoFiles+TestGoFiles files, absolute paths
+	gofiles []string // GoFiles+CgoFiles+TestGoFiles+XTestGoFiles files, absolute paths
 	target  string   // installed file for this package (may be executable)
 	fake    bool     // synthesized package
 }
@@ -134,6 +135,7 @@ func scanPackage(ctxt *build.Context, t *build.Tree, arg, importPath, dir string
 		Imports:      info.Imports,
 		GoFiles:      info.GoFiles,
 		TestGoFiles:  info.TestGoFiles,
+		XTestGoFiles: info.XTestGoFiles,
 		CFiles:       info.CFiles,
 		HFiles:       info.HFiles,
 		SFiles:       info.SFiles,
@@ -162,6 +164,9 @@ func scanPackage(ctxt *build.Context, t *build.Tree, arg, importPath, dir string
 	for _, f := range info.TestGoFiles {
 		p.gofiles = append(p.gofiles, filepath.Join(dir, f))
 	}
+	for _, f := range info.XTestGoFiles {
+		p.gofiles = append(p.gofiles, filepath.Join(dir, f))
+	}
 
 	sort.Strings(p.gofiles)