go.tools/go/loader: rename backdoor so only tests can use it.

Also: add copyright message.

LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/64040044

go/loader/backdoor_test.go

@@ -1,3 +1,7 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 package loader
 
 import (