[Perf] Update references to telemetry.core.discover

tools/perf/benchmarks/system_health_smoke_test.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.
 
 """Run all system health stories used by system health benchmarks.
 
 Only memory benchmarks are used when running these stories to make the total
 cycle time manageable. Other system health benchmarks should be using the same
 stories as memory ones, only with fewer actions (no memory dumping).
 """
 
 import unittest
 
 from core import perf_benchmark
 
 from telemetry import benchmark as benchmark_module
 from telemetry import decorators
-from telemetry.core import discover
 from telemetry.internal.browser import browser_finder
 from telemetry.testing import options_for_unittests
 from telemetry.testing import progress_reporter
 
+from py_utils import discover
+
 from benchmarks import system_health
 
 
 def GetSystemHealthBenchmarksToSmokeTest():
   sh_benchmark_classes = discover.DiscoverClassesInModule(
       system_health, perf_benchmark.PerfBenchmark,
       index_by_class_name=True).values()
   return list(b for b in sh_benchmark_classes if
               b.Name().startswith('system_health.memory'))
 
@@ skipping 138 matching lines @@
     # Prefetch WPR archive needed by the stories set to avoid race condition
     # when feching them when tests are run in parallel.
     # See crbug.com/700426 for more details.
     stories_set.wpr_archive_info.DownloadArchivesIfNeeded()
 
     for story_to_smoke_test in stories_set.stories:
       suite.addTest(
           _GenerateSmokeTestCase(benchmark_class, story_to_smoke_test))
 
   return suite