Move pub into sdk/lib/_internal.

sdk/lib/_internal/pub/test/install/relative_symlink_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS 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 file.
 
 library pub_tests;
 
 import 'dart:io';
 
 import 'package:scheduled_test/scheduled_test.dart';
 
@@ skipping 44 matching lines @@
       d.dir("bin", [
         d.dir("packages", [
           d.dir("myapp", [
             d.file('foo.dart', 'main() => "foo";')
           ])
         ])
       ])
     ]).validate();
   });
 }