This directory contains all Common Lisp dependencies.  They are fetched via Git
submodules.  This gives us good reproducibility and control, unlike with
Quicklisp which might not have the right versions.

We store these in a directory that's excluded from recursion by ASDF.
as per =asdf/source-registry:*source-registry-exclusions*=.
This way we won't conflict with user or system libraries.
