[Perf] Update references to telemetry.core.discover

tools/perf/contrib/cluster_telemetry/multipage_skpicture_printer.py

 # Copyright 2016 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.
 
 from core import perf_benchmark
 
 from contrib.cluster_telemetry import ct_benchmarks_util
 from contrib.cluster_telemetry import page_set
 from contrib.cluster_telemetry import repaint_helpers
 
-from telemetry.core import discover
 from telemetry import story
 
+from py_utils import discover
+
 from measurements import multipage_skpicture_printer
 
 
 def _MatchPageSetName(story_set_name, story_set_base_dir):
   story_sets = discover.DiscoverClasses(story_set_base_dir, story_set_base_dir,
                                         story.StorySet).values()
   for s in story_sets:
     if story_set_name == s.Name():
       return s
   return None
@@ skipping 62 matching lines @@
   def CreateStorySet(self, options):
     return page_set.CTPageSet(
         options.urls_list, options.user_agent, options.archive_data_file,
         run_page_interaction_callback=repaint_helpers.WaitThenRepaint)
 
   def GetExpectations(self):
     class StoryExpectations(story.expectations.StoryExpectations):
       def SetExpectations(self):
         pass # Not tests disabled.
     return StoryExpectations()