Reorganize barback's source files.

pkg/barback/lib/src/package_provider.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.package_provider;
 
 import 'dart:async';
 
-import 'asset.dart';
-import 'asset_id.dart';
+import 'asset/asset.dart';
+import 'asset/asset_id.dart';
 
 /// API for locating and accessing packages on disk.
 ///
 /// Implemented by pub and provided to barback so that it isn't coupled
 /// directly to pub.
 abstract class PackageProvider {
   /// The names of all packages that can be provided by this provider.
   ///
   /// This is equal to the transitive closure of the entrypoint package
   /// dependencies.
   Iterable<String> get packages;
 
   /// Loads an asset from disk.
   ///
   /// This should be re-entrant; it may be called multiple times with the same
   /// id before the previously returned future has completed.
   Future<Asset> getAsset(AssetId id);
 }