project: Isolate ManifestProject from RepoProject

Create RepoProject and ManifestProject, inheriting from MetaProject,
  with methods separated for isolation and clarity.

Change-Id: Ic1d6efc65c99470290fea612e2abaf8670d199f4
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/334139
Tested-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>