fonts: remove 'fonts.fontDir.enable' and use 'fonts.packages' (#439)

Remove the obsolete 'fonts.fontDir.enable' option and replace the
'fonts.fonts' option with 'fonts.packages'.

This should fix Stylix for nix-darwin past 58b905ea.

Closes: #435

stylix/darwin/fonts.nix

@@ -5,7 +5,6 @@ let
 in {
   imports = [ ../fonts.nix ];
   config.fonts = lib.mkIf config.stylix.enable {
-    fontDir.enable = true;
-    fonts = cfg.packages;
+    inherit (cfg) packages;
   };
 }