Reorganize barback's source files.

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

Index: pkg/barback/lib/src/transformer/base_transform.dart
diff --git a/pkg/barback/lib/src/base_transform.dart b/pkg/barback/lib/src/transformer/base_transform.dart
similarity index 96%
rename from pkg/barback/lib/src/base_transform.dart
rename to pkg/barback/lib/src/transformer/base_transform.dart
index bb0183c5d74be119b6a0358a6fc70774dda9b3b8..0672047f9919d643f717de1f896b602771a4356a 100644
--- a/pkg/barback/lib/src/base_transform.dart
+++ b/pkg/barback/lib/src/transformer/base_transform.dart
@@ -2,13 +2,13 @@
 // 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.base_transform;
+library barback.transformer.base_transform;
 
 import 'dart:async';
 
-import 'log.dart';
+import '../graph/transform_node.dart';
+import '../log.dart';
 import 'transform_logger.dart';
-import 'transform_node.dart';
 
 /// The base class for the ephemeral transform objects that are passed to
 /// transformers.