Restructure YAML package suitable for pub lish

utils/pub/pubspec.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;
 
 import 'package.dart';
 import 'source.dart';
 import 'source_registry.dart';
 import 'utils.dart';
 import 'version.dart';
-import 'yaml/yaml.dart';
+import 'yaml/lib/yaml.dart';
 
 /**
  * The parsed and validated contents of a pubspec file.
  */
 class Pubspec {
   /**
    * This package's name.
    */
   final String name;
 
@@ skipping 161 matching lines @@
     }
 
     source.validateDescription(description, fromLockFile: false);
 
     dependencies.add(new PackageRef(
         name, source, versionConstraint, description));
   });
 
   return dependencies;
 }