Let's simplify things by defining all modules in `./plugins`, `./modules`
and `./wrappers/modules`.
Instead of currying `pkgs` into a bootstrapping module, we can require
`defaultPkgs` be provided as a special arg.
This refactor allows us to completely remove `flake-modules/modules.nix`!
treefmt isn't actually using flake-root to find the flake's root; it
does it itself.
All we were doing was passing flake-root's default `projectRootFile` to
treefmt, which is `"flake.nix"`;
