Reorganize barback's source files.

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

Index: pkg/barback/lib/src/transformer/transform.dart
diff --git a/pkg/barback/lib/src/transform.dart b/pkg/barback/lib/src/transformer/transform.dart
similarity index 94%
rename from pkg/barback/lib/src/transform.dart
rename to pkg/barback/lib/src/transformer/transform.dart
index 268849d78a419d2faa567df217ffc3560e948854..809cd122683484ae84c717edd0da3e1b77b9d851 100644
--- a/pkg/barback/lib/src/transform.dart
+++ b/pkg/barback/lib/src/transformer/transform.dart
@@ -2,18 +2,18 @@
 // 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.transform;
+library barback.transformer.transform;
 
 import 'dart:async';
 import 'dart:convert';
 
-import 'asset.dart';
-import 'asset_id.dart';
-import 'asset_set.dart';
+import '../asset/asset.dart';
+import '../asset/asset_id.dart';
+import '../asset/asset_set.dart';
+import '../errors.dart';
+import '../graph/transform_node.dart';
+import '../utils.dart';
 import 'base_transform.dart';
-import 'errors.dart';
-import 'transform_node.dart';
-import 'utils.dart';
 
 /// While a [Transformer] represents a *kind* of transformation, this defines
 /// one specific usage of it on a set of files.