use deps_nix

[jurraca]

Deps_nix is better than mix2nix in that it can handle git expressions and uses Nix expressions for each dependency.

However, it is a mix dependency, not a standalone binary, so it would require an additional step for the user. Currently, the templates include mix2nix via nixpkgs.

Finally, it seems to handle Rust, but I haven't successfully compiled a dependency that requires elixir_make (i.e. a C compiler).