Stumpwm: Fix contrib modules, install stumpish.

This was caused by multiple things: First, the module-path was wrong in
the release. Second, when modules tried to load stumpwm, asdf searched
for its sources in /tmp/nix-build-*.

Both of these issues are fixed by a nix-specific patch that tells adsf
to *never* try to load stumpwm (and others) from the filesystem. This is
fine as those modules are already available in the image anyway.

We also refactor some stuff & clean up the build. Stumpish works now
too.