Automatically detect Nixpkgs inputs

[lucperkins]

• Make Nixpkgs keys optional
• Handle explicit keys case in match statement
• Provide method for automatically detecting Nixpkgs

[greg-hellings]

This is of interest to me, as my system config flake uses keys named nixstable and nixunstable to bring in two versions of the packages to use on different systems within the flake. Would love to see this feature enabled.