feature request: update multiple derivations in one package

[av-gal]

Nix-update currently doesn't have any way of knowing when a package has multiple derivations that need to be updated. I ran into this when packaging autobrr in nixos/nixpkgs#287593: autobrr consists of a frontend built with pnpm, which is incorporated into the application's binary when the backend is built as a Go module.

I don't feel like writing my own update script, and I don't think breaking the package up into two files makes sense here (since neither part can work independently of the other). To fix this issue in general, I believe that nix-update would need to support multiple derivations in one package, both of which have hashes that need to be updated.