This minimal implementation allows `nixpkgs.pkgs` to be defined, but
does not implement evaluating an instance from a pkgsPath when _not_
defined.
The `defaultPkgs` specialArg is dropped in favour of `nixpkgs.pkgs`
being defined. If it's not defined, an assertion is thrown.
In the future, a nixpkgs source path can be supplied, defaulting to the
flake's `inputs.nixpkgs`. Along with other `nixpkgs.*` options, this
will allow a `pkgs` instance to be evaluated within the module eval.
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.
