Please make it possible to pass fc-scan a set of separate fontconfig files

When building a font package in Fedora/RHEL, the packaged files are deployed in a staging directory in the buildroot, different from the syste default root

Therefore, to compute the font provides associated with the new package, fc-scan needs to consider:

1. the system fontconfig settings in the usual place
2. the new font files in the staging area
3. the new fontconfig files in the staging area

You can pass explicit font file paths to fc-scan, solving 2, but not explicit config files paths

Therefore, right now, rpm and appstream provides data is incorrect, because it registers the font caracteristics without the effect of the conf files in the same package

Also, as we've seen recently, it also breaks testing in koshei, because of the same problem (unability to process the config the package will deploy in addition to the font files it will deploy)

Lastly, the feature would be useful for testing new config files while they are being created, without needing to drop them in /etc/fonts or ~/.local/fonts. When you work on a set of new conf files needing to play with symlinks all the time to test various configuration combinations is awful.