Hi guix,

This patch defines an updater for minetest mods on ContentDB.

Example "./pre-inst-env guix refresh -t minetest" output:
gnu/packages/minetest.scm:368:13: minetest-mobs would be upgraded from 2021-07-22 to 2021-09-20
gnu/packages/minetest.scm:455:13: minetest-technic would be upgraded from 2021-04-15 to 2021-09-11
gnu/packages/minetest.scm:598:13: minetest-unified-inventory would be upgraded from 2021-03-25-1 to 2021-08-25

Only detecting updates is currently supported.  To actually
perform the uppdates, a patch like
<https://issues.guix.gnu.org/50072#4> is required.

Greetings,
Maxime.