Instead of exposing the entire package tree from nixpkgs, whitelist
individual packages explicitly so that they show up in
`pkgs.third_party`.
This makes it much easier to control external dependencies used by my
projects.
Bonus: It even includes a working `third_party.callPackage` with only
the whitelisted packages!
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