import mock from third_party explicitly

Stop relying on third_party/ leaking into sys.path all the time.

BUG=b:187794934
TEST=CQ passes

Change-Id: If233859063e6d2a1f2fdaa851febcbe8f536896a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2896053
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
diff --git a/api/controller/packages_unittest.py b/api/controller/packages_unittest.py
index 35ba7a9..de5e6c6 100644
--- a/api/controller/packages_unittest.py
+++ b/api/controller/packages_unittest.py
@@ -7,8 +7,6 @@
 
 from __future__ import print_function
 
-import mock
-
 from chromite.api.api_config import ApiConfigMixin
 from chromite.api.controller import controller_util
 from chromite.api.controller import packages as packages_controller
@@ -23,6 +21,7 @@
 from chromite.lib import uprev_lib
 from chromite.lib.parser import package_info
 from chromite.service import packages as packages_service
+from chromite.third_party import mock
 
 
 class UprevTest(cros_test_lib.MockTestCase, ApiConfigMixin):