xkbcomp: allow including kccgst files from other paths

xkbcomp: allow including kccgst files from other paths

Previously, a 'symbols/us' file in path A would shadow the same file in path B.
This is suboptimal, we rarely need to hide the system files - we care mostly
about *extending* them. By continuing to check other lookup paths, we make it
possible for a XDG_CONFIG_HOME/xkb/symbols/us file to have sections including
those from /usr/share/X11/xkb/symbols/us.

Note that this is not possible for rules files which need to be manually
controlled to get the right bits resolved.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>