Reorganize barback's source files.

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

Index: pkg/barback/lib/src/transformer/transform_logger.dart
diff --git a/pkg/barback/lib/src/transform_logger.dart b/pkg/barback/lib/src/transformer/transform_logger.dart
similarity index 96%
rename from pkg/barback/lib/src/transform_logger.dart
rename to pkg/barback/lib/src/transformer/transform_logger.dart
index 5f43a2dc44c02517d0746f521dee4982ed4f9c6f..652437571db16622f04b1d1c650279e000ccb192 100644
--- a/pkg/barback/lib/src/transform_logger.dart
+++ b/pkg/barback/lib/src/transformer/transform_logger.dart
@@ -2,12 +2,12 @@
 // 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_logger;
+library barback.transformer.transform_logger;
 
 import 'package:source_maps/span.dart';
 
-import 'asset_id.dart';
-import 'log.dart';
+import '../asset/asset_id.dart';
+import '../log.dart';
 
 typedef void LogFunction(AssetId asset, LogLevel level, String message,
                          Span span);