[STUMP] Load module hacks

Hi All,

tldr; New branch for testing called load-module-hacks. *contrib-dir* now
points to ~/.stumpwm.d/contrib, its fully recursive, and it may break
your config.

I got a chance to work over the module.lisp file to do a few things a
little better.  Specifically:
- Fully recursive traversal of *contrib-dir* 
- Removal of distribution dependent pathname functions
- *contrib-dir* now defaults to ~/.stumpwm.d/contrib <-- !! IMPORTANT !!
- Make target that allows "make get-contrib" which "git clone"s the
  contrib repo to ~/.stumpm.d/contrib

In a week or two I'm going to merge this in with the main branch.  It
will break/change behavior, so be on the lookout for the change when it
comes (I'll also send another email). 

I'll also be updating the manual/docs before I merge this into the main
branch. 

The branch for testing is here:
https://github.com/stumpwm/stumpwm/tree/load-module-hacks

Cheers,

    Dave