This is not the final layout yet, but makes it so that my top-level
attribute set is no longer overlaid into nixpkgs itself.
This is useful for other people who are importing my monorepo.
Broadly speaking, the following things are included:
* there is now a uniform `args` struct that is passed to all
derivations, package headers have been changed appropriately
* overrides are now loaded from a separate `override` folder just
using read-tree.nix
* third-party packages have moved into the `third_party` attribute set
This pulls in an old version of Elm from NixOS 17.09 which can still
build the Elm code in Gemma.
However, the Common Lisp build is now broken in some other way.
