Remove dependency on DepotDownloader

[AlphaKeks]

1. shelling out is gross
2. it's yet another dependency you have to install
3. it doesn't play nice with immutable file systems (e.g. docker images built by nix)

Ideally we implement the functionality we need (downloading workshop maps) directly as a Rust library, but it isn't obvious to me how DepotDownloader works just by looking at their code...