On Fri, Jan 27 2017, Ludovic Courtès wrote
> What needs to be done?  We cannot guess the upstream URL just from the
> MELPA one, can we?

No, we can't just get it from the URL, but MELPA's recipes should give
us enough information to be able to build them. (After all, they give
MELPA enough information to be able to build them.)

https://github.com/melpa/melpa/tree/master/recipes

I think this is where they're actually processed:

https://github.com/melpa/melpa/blob/8babd80660a50183d81a9269e83c9e473c450ffb/package-build/package-build.el

Carlo