Reorganize barback's source files.

pkg/barback/lib/src/lazy_transformer.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 barback.lazy_transformer;
-
-import 'declaring_transformer.dart';
-
-/// An interface for [Transformer]s that indicates that the transformer's
-/// outputs shouldn't be generated until requested.
-///
-/// The [declareOutputs] method is used to figure out which assets should be
-/// treated as "lazy." Lazy assets will only be forced to be generated if
-/// they're requested by the user or if they're used by a non-declaring
-/// transformer.
-abstract class LazyTransformer extends DeclaringTransformer {}