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/compile_build_api_proto_unittest.py b/api/compile_build_api_proto_unittest.py
index 135ca0a..751fcf4 100644
--- a/api/compile_build_api_proto_unittest.py
+++ b/api/compile_build_api_proto_unittest.py
@@ -4,7 +4,7 @@
 
 """compile_build_api_proto tests."""
 
-from google import protobuf
+from chromite.third_party.google import protobuf
 from chromite.api import compile_build_api_proto