Move PackageName stuff out of package.dart.

These classes are largely independent of the Package class.
diff --git a/lib/src/pubspec.dart b/lib/src/pubspec.dart
index 18d0b5c..775f6cb 100644
--- a/lib/src/pubspec.dart
+++ b/lib/src/pubspec.dart
@@ -11,7 +11,7 @@
 import 'compiler.dart';
 import 'exceptions.dart';
 import 'io.dart';
-import 'package.dart';
+import 'package_name.dart';
 import 'source_registry.dart';
 import 'utils.dart';