Create a third_party subdirectory and a third_party/lisp. This directory layout
resembles and is inspired by the layout of Google's mono-repo, Google3. @tazjin
borrowed this idea from Google and I'm borrowing the idea from him.
My first foray trying to package Common Lisp with Nix. I'm using @tazjin's
buildLisp and other libraries, all of which I'm importing as `tpkgs`, and all of
which have been a tremendous boon to my productivity.