In the two places where our "helpers" lib is exclusively internal
(flake module args and building the docs), we no longer supply `pkgs`.
In the other 4 locations, we now note why we still do in a comment.
Rather than nesting it as `config`, pass `makeNixvim`'s argument through
directly as a module.
This not only simplifies the implementation, but allows users to use
`makeNixvim` for any scenario where `makeNixvimWithModule` would normally
be required.
