Use pub_semver package in pub.

sdk/lib/_internal/pub/lib/src/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 pub.pubspec;
 
 import 'package:path/path.dart' as path;
+import 'package:pub_semver/pub_semver.dart';
 import 'package:source_span/source_span.dart';
 import 'package:yaml/yaml.dart';
 
 import 'barback/transformer_config.dart';
 import 'exceptions.dart';
 import 'io.dart';
 import 'package.dart';
 import 'source_registry.dart';
 import 'utils.dart';
-import 'version.dart';
 
 /// The parsed contents of a pubspec file.
 ///
 /// The fields of a pubspec are, for the most part, validated when they're first
 /// accessed. This allows a partially-invalid pubspec to be used if only the
 /// valid portions are relevant. To get a list of all errors in the pubspec, use
 /// [allErrors].
 class Pubspec {
   // If a new lazily-initialized field is added to this class and the
   // initialization can throw a [PubspecException], that error should also be
@@ skipping 551 matching lines @@
     implements ApplicationException {
   PubspecException(String message, SourceSpan span)
       : super(message, span);
 }
 
 /// Returns whether [uri] is a file URI.
 ///
 /// This is slightly more complicated than just checking if the scheme is
 /// 'file', since relative URIs also refer to the filesystem on the VM.
 bool _isFileUri(Uri uri) => uri.scheme == 'file' || uri.scheme == '';