Fix pylint issue in payload.py

    ************* Module update_payload.payload
    W: 12, 0: Relative import 'applier', should be 'update_payload.applier' (relative-import)
    W: 13, 0: Relative import 'block_tracer', should be 'update_payload.block_tracer' (relative-import)
    W: 14, 0: Relative import 'checker', should be 'update_payload.checker' (relative-import)
    W: 15, 0: Relative import 'common', should be 'update_payload.common' (relative-import)
    W: 16, 0: Relative import 'error', should be 'update_payload.error' (relative-import)
    W: 17, 0: Relative import 'update_metadata_pb2', should be 'update_payload.update_metadata_pb2' (relative-import)
    C:222, 4: Missing function docstring (missing-docstring)
    E:261,33: Instance of 'DeltaArchiveManifest' has no 'partitions' member (no-member)

Test: repo upload
Change-Id: Ib3e99c6a3a5ea4575b442e672a871adc7b985289
(cherry picked from commit c2527f49)
Reviewed-on: https://chromium-review.googlesource.com/834874


Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>