`lib.evalModules` always includes `lib`, `config`, `options`, &
(configured) `specialArgs` in the (final) `specialArgs`[1].
Therefore, setting `_module.args.lib` has no effect.
[1]: 329d232802/lib/modules.nix (L233)
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`!