Runtime determination of share directories?

Currently maxima has a manually maintained list of share directories
(see set-pathnames in src/init-cl.lisp). 

Wouldn't it be nicer if we could do this at startup and just find all of
the directories in share directory?  Then we wouldn't have to maintain
this list at all, and users can easily add share packages themselves and
have them automatically available.  (Well, perhaps we could keep a few
of the core share packages in the list, but most would be determined at
runtime).

I played with this a while back, but never finished it.  I think,
however, that all of the lisps we support can do this using the Lisp
DIRECTORY function with a bit of extra glue to return the right stuff. 
GCL might be a bit more difficult, but I think it can be done there too.

Anyway, just a thought.

Ray