keyboard: Move code to get the XkbRF_VarDefsRec here

GnomeXkbInfo needs to drop its X dependency to be used on a wayland
compositor gnome-shell and in that case the xkb data path is an
implementation detail.

This g-s-d plugin will still be used on X only setups so we'll move
the code here.

https://bugzilla.gnome.org/show_bug.cgi?id=724277