repo: Add support for standalone manifests

Added --standalone_manifest to repo tool. If set, the
manifest is downloaded directly from the appropriate source
(currently, we only support GS) and used instead of creating
a manifest git checkout. The manifests.git repo is still created to
keep track of various config but is marked as being for a standalone
manifest so that the repo tool doesn't try to run networked git
commands in it.

BUG=b:192664812
TEST=existing tests (no coverage), manual runs

Change-Id: I84378cbc7f8e515eabeccdde9665efc8cd2a9d21
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/312942


Tested-by: Jack Neus <jackneus@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>