Use pub_semver package in pub.

sdk/lib/_internal/pub/lib/src/cached_package.dart

 // Copyright (c) 2014, 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.cached_package;
 
 import 'package:path/path.dart' as p;
+import 'package:pub_semver/pub_semver.dart';
 import 'package:yaml/yaml.dart';
 
 import 'barback/transformer_config.dart';
 import 'io.dart';
 import 'package.dart';
 import 'pubspec.dart';
-import 'version.dart';
 
 /// A [Package] whose `lib` directory has been precompiled and cached.
 ///
 /// When users of this class request path information about files that are
 /// cached, this returns the cached information. It also wraps the package's
 /// pubspec to report no transformers, since the transformations have all been
 /// applied already.
 class CachedPackage extends Package {
   /// The directory contianing the cached assets from this package.
   ///
@@ skipping 52 matching lines @@
   String get publishTo => _inner.publishTo;
   Map<String, String> get executables => _inner.executables;
   bool get isPrivate => _inner.isPrivate;
   bool get isEmpty => _inner.isEmpty;
   List<PubspecException> get allErrors => _inner.allErrors;
 
   List<Set<TransformerConfig>> get transformers => const [];
 
   _CachedPubspec(this._inner);
 }