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
The port and resource directory are now specified via environment
variables and a wrapper script is created by Nix that sets the
resource path and so on correctly.
