[Perf] Update references to telemetry.core.discover

tools/chrome_proxy/integration_tests/chrome_proxy_pagesets/__init__.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import inspect
 import os
 import sys
 
-from telemetry.core import discover
 from telemetry import story
 import video
 
+from py_utils import discover
 
 # Import all submodules' StorySet classes.
 start_dir = os.path.dirname(os.path.abspath(__file__))
 top_level_dir = os.path.abspath(os.path.join(start_dir, os.pardir, os.pardir))
 base_class = story.StorySet
 for cls in discover.DiscoverClasses(
     start_dir, top_level_dir, base_class).values():
   setattr(sys.modules[__name__], cls.__name__, cls)
 
 # DiscoverClasses makes the assumption that there is exactly one matching
 # class per file, however the following are declared in the same file.
 for cls in (video.VideoDirectStorySet,
             video.VideoProxiedStorySet,
             video.VideoCompareStorySet):
   setattr(sys.modules[__name__], cls.__name__, cls)