Previously, service descriptions appeared in both guix.pot and
packages.pot, but only translations of packages.pot were used.
Now, translations are only done with guix.pot.
This is better, because translators are more likely to translate
guix.pot, and is also easier, because files in gnu/{home/,}services
need to be in po/guix/POTFILES.in anyway and po/guix/Makevars
already acts on the 'description' keyword because of lint checkers.
* guix/scripts/home.scm (service-type-description-string):
Use translations from guix.pot.
* guix/scripts/system/search.scm (service-type-description-string)
(service-type->recutils): Likewise.
* po/packages/POTFILES.in: Move files in gnu/services to ...
* po/guix/POTFILES.in: ... here.
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