Falling back to unversioned `/etc/fonts/conf.d` when versioned one does not exist
is problematic since it only occurs on non-NixOS systems and those are likely
to have a different version of fontconfig. When those versions use incompatible
elements in the config, apps using fontconfig will crash.

Instead, we are now falling back to the in-package `fonts.conf` file that loads
both the versioned global `conf.d` directory and the in-package `conf.d` since using
upstream settings on non-NixOS is preferable to not being able to use apps there.

In fact, we would not even need to link `fonts.conf`, as the in-package `fonts.conf`
will be always used unless someone creates the global one manually (the option is still
retained if one wants to write a custom NixOS module and to avoid unnecessary stat call on NixOS).

Additionally, since the `fonts.conf` will always load `conf.d` from the package, we no longer
need to install them to sytem `/etc` in the module. This needed some mucking with `50-user.conf`
which disables configs in user directories (a good thing IMO, NixOS module will turn it back on)
but otherwise, it is cleaner. The files are still prioritized by their name, regardless of their location.

See #73795 (comment) for more information.

With previous patch, we no longer load non-versioned fonts.conf file to avoid incompatibilities
but this also means fontconfig will not load system-wide installed fonts on non-NixOS systems.

As a compromise, let's hardcode the FHS font paths to the built-in config so that the system
fonts work there. Unlike with the system config we do not need to worry about compatibility as
incompatible font files will be simply ignored.

Of course there will still be disparity if the system install fonts to some other location than
these two but I am afraid this is the best we can do.

See #73795 (comment) for discussion.

ITS rules are used for extracting translatable strings and they have
been moved to external files in 2.13.92 so they are not needed in
the config files themselves.

Removing them also cuts down on errors/warnings produced when using
older versions of fontconfig (< 2.12.92). Now it will only complain
about the description element but that is fortunately just a warning,
not errors like the ones caused by the its attributes.

Thanks to this, we can change the config version in NixOS module
back to 2.11 allowing us to re-use the 2.13/2.14 configs for apps
built against 2.12 fontconfig.