Reorganize barback's source files.

pkg/barback/lib/src/transformer/transformer.dart

 // Copyright (c) 2013, 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.transformer;
+library barback.transformer.transformer;
 
 import 'dart:async';
 
-import 'asset_id.dart';
+import '../asset/asset_id.dart';
+import '../utils.dart';
 import 'transform.dart';
-import 'utils.dart';
 
 /// A [Transformer] represents a processor that takes in one or more input
 /// assets and uses them to generate one or more output assets.
 ///
 /// Dart2js, a SASS->CSS processor, a CSS spriter, and a tool to concatenate
 /// files are all examples of transformers. To define your own transformation
 /// step, extend (or implement) this class.
 ///
 /// If possible, transformers should implement [DeclaringTransformer] as well to
 /// help barback optimize the package graph.
@@ skipping 62 matching lines @@
   /// In other words, a Transformer's job is to find all inputs for a
   /// transform, starting at the primary input, then generate all output assets
   /// and yield them back to the transform.
   ///
   /// If this does asynchronous work, it should return a [Future] that completes
   /// once it's finished.
   apply(Transform transform);
 
   String toString() => runtimeType.toString().replaceAll("Transformer", "");
 }