Move pub into sdk/lib/_internal.

sdk/lib/_internal/pub/test/install/path/relative_path_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE d.file.
 
 import 'package:pathos/path.dart' as path;
 
-import '../../../../pub/exit_codes.dart' as exit_codes;
+import 'package:pub/src/exit_codes.dart' as exit_codes;
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration("can use relative path", () {
     d.dir("foo", [
       d.libDir("foo"),
       d.libPubspec("foo", "0.0.1")
     ]).create();
@@ skipping 46 matching lines @@
     d.dir(packagesPath, [
       d.dir("foo", [
         d.file("foo.dart", 'main() => "foo";')
       ]),
       d.dir("bar", [
         d.file("bar.dart", 'main() => "bar";')
       ])
     ]).validate();
   });
 }