* xdg-mime: allow overrides to shared-mime-info and desktop-file-utils
The `xdg-mime` module now exposes packages to determine what will be run
for update-mime-database and update-desktop-database. This allows users
to select a different version of these packages if the are incompatible.
This should, in combination with an override to the version of
`shared-mime-info` (can be found here notalltim/home-manager-config#4),
resolve#4955, #5102, #4682, and possibly #4941. The problem seems to stem
from a mismatch in the version of `shared-mime-info` with the host.
I also switched from using `buildPackages` to `pkgs` to improve
cross-compilation compatibility.
* xdg-mime: Add tests for xdg-mime module
The xdg-mime module was missing tests so I added basic test for all the
options and checked the basic behavior. It covers ensuring that the
proper files/folders are created and that the package overrides work.
