fontconfig: Only read versioned config dirs

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 https://github.com/NixOS/nixpkgs/pull/73795#issuecomment-634370125 for more information.

nixos/modules/config/fonts/fontconfig.nix

@@ -214,15 +214,7 @@ let
     # fontconfig default config files
     ln -s ${supportPkg.out}/etc/fonts/conf.d/*.conf \
           $support_folder/
-    ln -s ${latestPkg.out}/etc/fonts/conf.d/*.conf \
-          $latest_folder/
-
-    # update latest 51-local.conf path to look at the latest local.conf
-    rm    $latest_folder/51-local.conf
-
-    substitute ${latestPkg.out}/etc/fonts/conf.d/51-local.conf \
-               $latest_folder/51-local.conf \
-               --replace local.conf /etc/fonts/${latestVersion}/local.conf
+    # Latest fontconfig is configured to look for the upstream defaults inside the package.
 
     # 00-nixos-cache.conf
     ln -s ${cacheConfSupport} \
@@ -236,7 +228,11 @@ let
     # 50-user.conf
     ${optionalString (!cfg.includeUserConf) ''
     rm $support_folder/50-user.conf
-    rm $latest_folder/50-user.conf
+    ''}
+    # Since latest fontconfig looks for default files inside the package,
+    # we had to move this one elsewhere to be able to exclude it here.
+    ${optionalString cfg.includeUserConf ''
+    ln -s ${latestPkg.out}/etc/fonts/conf.d.bak/50-user.conf $latest_folder/50-user.conf
     ''}
 
     # local.conf (indirect priority 51)