Expose our locked nixpkgs as the `nixpkgs.source` module options.
This only happens when `evalNixvim` is part of a lib that was provided
`flake` as an argument.
Stubbed the `nixpkgs.source` option for now. Eventually, this will be
used to construct `pkgs` internally. For now, it's purely informational.
Based on the `nixpkgs.overlays` option available in NixOS, allows users
to further customize the `pkgs` instance used when evaluating nixvim.
The standard module tests are now provided a few extra module args to
enable a test where we instantiate a custom nixpkgs instance.
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.
