Rewrite ExtensionDistributor to no longer require working copies

    Instead, allow it to fetch tars from remote services (such as github)
    that allow us to generate tar snapshots per-branch

Change-Id: I9d3c328f1994d13f9c17b6e17b68f6b964c211aa
Merge "Rewrite ExtensionDistributor to no longer require working copies"