Handle priority in epl-package-version

[Silex]

Fix #32

@fommil: can you test that this fixes it?

[fommil]

nope, this deleted a bunch of packages then seemed to try to reinstall some things without their deps available.

[Silex]

Very weird, it's one of these case where I can't see why it would not work.

I mean, it's what package.el uses:

(defun package-archive-priority (archive)
  "Return the priority of ARCHIVE.

The archive priorities are specified in
`package-archive-priorities'. If not given there, the priority
defaults to 0."
  (or (cdr (assoc archive package-archive-priorities))
      0))

(defun package-desc-priority-version (pkg-desc)
  "Return the version PKG-DESC with the archive priority prepended.

This allows for easy comparison of package versions from
different archives if archive priorities are meant to be taken in
consideration."
  (cons (package-desc-priority pkg-desc)
        (package-desc-version pkg-desc)))

I guess the fact that package.el also deletes the old package has to be taken in account... That or you tested wrongly 😜

I'll try to test by actually using package-archive-priorities when I have time.

[Silex]

@lunaryorn: I realised that implementing around package.el was actually pretty simple, so I did it. I don't depend on epl anymore.

This PR should be close to the right thing to do, but apparently it isn't, and I don't feel like debugging it further.

Feel free to close or keep it open as a future TODO.