[Perf] Update references to telemetry.core.discover

tools/perf/core/benchmark_finders.py

 # Copyright 2015 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 imp
 import inspect
 import os
 import sys
 
 from core import path_util
 from core import perf_benchmark
 
-from telemetry.core import discover
 from telemetry import benchmark as benchmark_module
 
+from py_utils import discover
 
 def GetClassFilePath(clazz):
   """ Return the absolute file path to |clazz|. """
   assert inspect.isclass(clazz)
   path = os.path.abspath(inspect.getfile(clazz))
   if path.endswith('.pyc'):
     return path[:-1]
   return path
 
 
@@ skipping 37 matching lines @@
   all_perf_benchmarks = GetAllPerfBenchmarks()
   all_contrib_benchmarks = GetAllContribBenchmarks()
   return all_perf_benchmarks + all_contrib_benchmarks
 
 def GetBenchmarksInSubDirectory(directory):
   return discover.DiscoverClasses(
     start_dir=directory,
     top_level_dir = path_util.GetPerfDir(),
     base_class=benchmark_module.Benchmark,
     index_by_class_name=True).values()