Tracking upstream releases

[Ludovic Courtès]

Hello!

We now have a number of importers, which is great, but so far we had no
support to update packages once they had been imported, even though most
of the code to do that already is in the importer.

So commit 0a7c5a09 generalizes the upstream-tracking code from (guix
gnu-maintenance) a bit, such that we can then easily define “updaters”
based on the import code.  As an example, subsequent commits add an
updater for ELPA and one for CRAN.  The end result is:

--8<---------------cut here---------------start------------->8---
$ ./pre-inst-env guix refresh -t cran -t elpa
gnu/packages/statistics.scm:819:13: r-testthat would be upgraded from 0.10.0 to 
0.11.0
gnu/packages/emacs.scm:856:13: emacs-auctex would be upgraded from 11.88.6 to 
11.88.9
--8<---------------cut here---------------end--------------->8---

(When ‘-t’ is omitted, all the updaters are used.)

So, I invite you to add an updater for your favorite importer!
It’s easy and super cool!  :-)

Ludo’.