Add Fetchcode support for Nix

Nix is not a central repository, packages are hosted in different locations, making it difficult to determine their direct download URLs.
However, we can determine and construct a direct download URL based on the package's homepage and version.

Initially, we should provide a few well‑known repositories and construct direct download URLs when packages are hosted there.

For all other hosts, the `download_url` will return None.
Additional repositories will be supported progressively.