Maximize code reuse between nixpkgs-compatible expression and overlay

Yes, I had come across it in the past, and my attempts were, indeed, influenced by it.

Sigh. Flakes.

Yes, I’ve been wanting to use them for some time, but I’ve been holding off because their unstable status and my limited time available for all things Nix, didn’t seem like a good fit.

But maybe it’s time to take the plunge.