Make scripts/update_payload available for other python modules

The new merge_ota tool will need this module to read/write OTA packages

Test: th
Bug: 227848550

Change-Id: Ibfe2fca65ffbaca939ea3f8ca924c9152e5e7540
diff --git a/scripts/update_payload/checker_unittest.py b/scripts/update_payload/checker_unittest.py
index 993b785..cf813fd 100755
--- a/scripts/update_payload/checker_unittest.py
+++ b/scripts/update_payload/checker_unittest.py
@@ -37,7 +37,7 @@
 from update_payload import checker
 from update_payload import common
 from update_payload import test_utils
-from update_payload import update_metadata_pb2
+import update_metadata_pb2
 from update_payload.error import PayloadError
 from update_payload.payload import Payload  # Avoid name conflicts later.