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 '../../pkg/yaml/lib/yaml.dart';
 
 /// The parsed and validated contents of a pubspec file.
 class Pubspec {
   /// This package's name.
   final String name;
 
   /// This package's version.
   final Version version;
 
   /// The packages this package depends on.
@@ skipping 186 matching lines @@
 /// The environment-related metadata in the pubspec. Corresponds to the data
 /// under the "environment:" key in the pubspec.
 class PubspecEnvironment {
   /// The version constraint specifying which SDK versions this package works
   /// with.
   final VersionConstraint sdkVersion;
 
   PubspecEnvironment([VersionConstraint sdk])
       : sdkVersion = sdk != null ? sdk : VersionConstraint.any;
 }