google.protobuf: move to explicit import

We want to pull third_party out of implicit sys.path search,
so change importing of these modules to explicit.

BUG=b:187794934
TEST=CQ passes

Change-Id: Ia247eb5214a984fbdc52504e4d6326a14474f05e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2917738
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
diff --git a/api/validate.py b/api/validate.py
index b017870..42c413a 100644
--- a/api/validate.py
+++ b/api/validate.py
@@ -14,7 +14,7 @@
 import os
 from typing import Callable, Iterable, List, Optional, Union
 
-from google.protobuf import message as protobuf_message
+from chromite.third_party.google.protobuf import message as protobuf_message
 
 from chromite.lib import cros_build_lib
 from chromite.lib import cros_logging as logging