Since starting to use `lib.mkPackageOption` throughout nixvim, users
will see a slightly different error message when encountering mismatched
nixpkgs channel errors.
Also fleshed out some recommendations.
Instead of maintainers providing an actual `defaultPackage`, they should
specify the pkg name which we'll use when calling `lib.mkPackageOption`.
This makes `mkVimPlugin` and `mkNeovimPlugin` compliant with #1950.
