[Perf] Update references to telemetry.core.discover

tools/perf/page_sets/__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
 from telemetry.story import expectations
 
+from py_utils import discover
 
 # Import all submodules' PageSet classes.
 start_dir = os.path.dirname(os.path.abspath(__file__))
 top_level_dir = os.path.dirname(start_dir)
 base_classes = [story.StorySet, expectations.StoryExpectations]
 
 for base_class in base_classes:
   for cls in discover.DiscoverClasses(
       start_dir, top_level_dir, base_class).values():
     setattr(sys.modules[__name__], cls.__name__, cls)