Reducing useless module imports

[Julien Lepiller]

Hi Guix!

during the Guix Days, we talked about reducing module imports. First,
let's note there's an empty module (gnu packages key-mon).

Then, we looked at build systems modules that are imported, but for
which the build system is never used:

```
for f in *.scm; do systems=$(grep '#:use-module (guix build-system' $f
| sed 's|^.*use-module (guix build-system \([^)]*\).*|\1|'); for s in
$systems; do if grep "${s}-build-system" $f >/dev/null; then :; else
echo $s is not used in $f; fi; done; done
```

For instance, (gnu packages minetest) imports (guix build-system
minetest) but never uses the minetest-build-system. (gnu packages
abiword) uses (guix build-system gnu) but does not use the
gnu-build-system.

Many modules seem to use a lot of imports that are actually useless,
for instance (gnu packages heads) imports 20 package modules, but
only uses 4 of them.

Removing these module imports would help a lot with reducing the number
of loops. If anyone wants to volunteer a lint checker for that,
raise hands :D