This looks really fragile to maintain.

I based it on how Swift itself is built - https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/compilers/swift/default.nix.

However another approach would be to just allow Swift Package Manager to fetch all dependencies from git repositories as in original code before patching. This would simplify the derivation but we would lose caching that Nix provides in this case. I am not sure how big of a problem it is, maybe it makes more sense to keep the derivation simple.

Here's my shot at packaging carthage using code from https://euandreh.xyz/swift2nix.git:

https://github.com/bmwalters/nixpkgs/tree/carthage/pkgs/development/mobile/carthage/default.nix

It avoids hardcoding dependencies while still fetching them through nix. It has several unsolved problems before it's ready for nixpkgs though. For example it may be cleaner to rely on editing Package.swift to point to local checkouts of dependencies as you've done here and as is done in this other swift2nix: https://github.com/zachcoyle/swift2nix.