Move pub into sdk/lib/_internal.

sdk/lib/_internal/pub/lib/src/validator/license.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 pubspec_field_validator;
 
 import 'dart:async';
 
 import 'package:pathos/path.dart' as path;
 
@@ skipping 17 matching lines @@
         return;
       }
 
       errors.add(
           "You must have a COPYING or LICENSE file in the root directory.\n"
           "An open-source license helps ensure people can legally use your "
           "code.");
     });
   }
 }