Reorganize barback's source files.

pkg/barback/lib/src/transformer/transformer_group.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_group;
+library barback.transformer.transformer_group;
 
 /// A [TransformerGroup] encapsulates a phased collection of transformers.
 ///
 /// A transformer group is defined like a collection of phases, such as you
 /// might pass to [Barback.updateTransformers]. Input assets are transformed by
 /// the first phase, whose results are passed to the second phase, and so on.
 ///
 /// However, from the perspective of someone using a [TransformerGroup], it
 /// works just like a [Transformer]. It can be included in phases, and will run
 /// in parallel to other transformers or groups in those phases. Other
 /// transformers and groups will be unable to see any intermediate assets that
 /// are generated by one phase of the group and consumed by another. Phases
 /// after the one containing the group will be able to see its outputs, though.
 class TransformerGroup {
   /// The phases that comprise this group.
   ///
   /// Each element of the inner iterable must be either a [Transformer] or a
   /// [TransformerGroup].
   final Iterable<Iterable> phases;
 
   TransformerGroup(Iterable<Iterable> phases)
       : this.phases = phases.map((phase) => phase.toList()).toList();
 
   String toString() => "group of $phases";
 }