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

tools/testing/dart/browser_test.dart

 // Copyright (c) 2011, 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.
 
 String getHtmlContents(String title,
                        Path controllerScript,
                        Path dartJsScript,
                        String scriptType,
                        Path sourceScript) =>
 """
@@ skipping 37 matching lines @@
 String wrapDartTestInLibrary(Path test) =>
 """
 library libraryWrapper;
 part '$test';
 """;
 
 String dartTestWrapper(Path dartHome, Path library) =>
 """
 library test;
 
-import '${dartHome.append('pkg/unittest/unittest.dart')}' as unittest;
-import '${dartHome.append('pkg/unittest/html_config.dart')}' as config;
+import '${dartHome.append('pkg/unittest/lib/unittest.dart')}' as unittest;
+import '${dartHome.append('pkg/unittest/lib/html_config.dart')}' as config;
 import '${library}' as Test;
 
 main() {
   config.useHtmlConfiguration();
   unittest.group('', Test.main);
 }
 """;