A more forwards-compatible file format would be defined that would
uncouple the format used by elpa-admin.el and package-vc.el the more I
think about this, the more I think it is better to take this path
instead of sharing code between the two.  Sure, this means that some
things have to be duplicated, but we are talking about two different
scenarios (activating source controlled code and packaging tarballs)
that might have a lot in common but still ought to be considered
separately.