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.
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