nixos-option: add backwards compatibility layer

This adds a basic `nixos-option/default.nix` file to provide some
measure of backwards compatibility with Nixpkgs prior to
ce6f17f9530ea013d267e07d2dd958bb4e3019dd.

Changed files
+1
nixos
modules
installer
tools
nixos-option
+1
nixos/modules/installer/tools/nixos-option/default.nix
···
+
{ pkgs, ... }: pkgs.nixos-option