Create mock pub links in the build for all packages under pkg, not a hard-coded list

pkg/pkg.gyp

 # 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.
 
 {
   'targets': [
     {
       'target_name': 'pkg_packages',
       'type': 'none',
       'actions': [
         {
           'action_name': 'make_pkg_packages',
           'inputs': [
-            '../tools/make_links.py',
-            'args/lib',
-            'fixnum/lib',
-            'htmlescape/lib',
-            'http/lib',
-            'intl/lib',
-            'logging/lib',
-            'meta/lib',
-            'unittest/lib',
-            'webdriver/lib',
+            '<!@(["python", "../tools/list_pkg_directories.py"])',
           ],
           'outputs': [
             '<(PRODUCT_DIR)/packages',
           ],
           'action': [
             'python', '../tools/make_links.py',
             '<(PRODUCT_DIR)/packages',
             '<@(_inputs)',
           ],
         },
       ],
     }
   ],
 }