Move pub into sdk/lib/_internal.

sdk/lib/_internal/pub/test/install/path/no_pubspec_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 'dart:io';
 
 import 'package:pathos/path.dart' as path;
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../../descriptor.dart' as d;
@@ skipping 13 matching lines @@
           "foo": {"path": fooPath}
         }
       })
     ]).create();
 
     schedulePub(args: ['install'],
         error: new RegExp('Package "foo" doesn\'t have a pubspec.yaml file.'),
         exitCode: 1);
   });
 }