Restructure pkg/unittest and pkg/webdriver to follow the pub conventions.

utils/tests/pub/install/git/check_out_branch_test.dart

 // Copyright (c) 2012, 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 pub_tests;
 
 import 'dart:io';
 
 import '../../test_pub.dart';
-import '../../../../../pkg/unittest/unittest.dart';
+import '../../../../../pkg/unittest/lib/unittest.dart';
 
 main() {
   test('checks out a package at a specific branch from Git', () {
     ensureGit();
 
     var repo = git('foo.git', [
       libDir('foo', 'foo 1'),
       libPubspec('foo', '1.0.0')
     ]);
     repo.scheduleCreate();
@@ skipping 11 matching lines @@
 
     dir(packagesPath, [
       dir('foo', [
         file('foo.dart', 'main() => "foo 1";')
       ])
     ]).scheduleValidate();
 
     run();
   });
 }