Re-introduced "package" archiving, fixes fork support for core

Pull request branches weren't found because the composer.json
didn't include those custom repos, which isn't possible without
using the original branch name (e.g. 'dev-pull/my-pr'),
which then conflicts with "require" constraints like '3.0.x'.
Could also be resolved through a "branch-alias" in all core
branches, but that still slowdowns because composer needs
to scan all existing branches on this new repo.