localed: match converted keymaps before legacy

Before, X11 keymap fr-pc105-oss would be converted to fr,
even though fr-oss exists. Now, if
/usr/lib/kbd/keymaps/xkb/<layout>[-<variant>].map[.gz] exists,
<layout>[-<variant>] will be used as the console keymap,
falling back to the legacy mappings otherwise.

% sudo localectl set-x11-keymap pl pc105
% localectl
   System Locale: LANG=en_US.UTF-8
       VC Keymap: pl                      (was pl2 before)
      X11 Layout: pl
       X11 Model: pc105
% sudo localectl set-x11-keymap fr pc105 oss
% localectl
   System Locale: LANG=en_US.UTF-8
       VC Keymap: fr-oss                  (was fr before)
      X11 Layout: fr
       X11 Model: pc105
     X11 Variant: oss
% sudo localectl set-x11-keymap fr pc105
% localectl
   System Locale: LANG=en_US.UTF-8
       VC Keymap: fr
      X11 Layout: fr
       X11 Model: pc105
% sudo localectl set-x11-keymap gb
% localectl
   System Locale: LANG=en_US.UTF-8
       VC Keymap: gb                     (was uk before)
      X11 Layout: gb