[Perf] Update references to telemetry.core.discover

tools/perf/measurements/measurement_smoke_test.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 logging
 import optparse
 import os
 import unittest
 
 from telemetry import benchmark as benchmark_module
-from telemetry.core import discover
 from telemetry.internal.browser import browser_options
 from telemetry.page import legacy_page_test
 from telemetry.testing import options_for_unittests
 from telemetry.web_perf import timeline_based_measurement
 
+from py_utils import discover
+
 
 def _GetAllPossiblePageTestInstances():
   page_test_instances = []
   measurements_dir = os.path.dirname(__file__)
   top_level_dir = os.path.dirname(measurements_dir)
   benchmarks_dir = os.path.join(top_level_dir, 'benchmarks')
 
   # Get all page test instances from measurement classes that are directly
   # constructible
   all_measurement_classes = discover.DiscoverClasses(
@@ skipping 27 matching lines @@
       page_test_instances.append(pt)
 
   return page_test_instances
 
 
 class MeasurementSmokeTest(unittest.TestCase):
   # Simple smoke test to make sure that all page_test are constructible.
 
   def testAllMeasurementInstance(self):
     _GetAllPossiblePageTestInstances()