Creating vendor directories directly in srcs of go and rust packages so fixed output derivations won't be needed

[danieldk]

@DavHau

The reason why mach-nix can do what it can do, is basically because it allows IFD.
IFD makes many things much much easier, as already mentioned by some people in this thread.

Just wanted to mention that even though the default mode of using crate2nix is through a pre-generated Cargo.nix, it also supports IFD. (I have been using that in CI for a while for Rust projects where I don’t want to keep Cargo.nix in sync with Cargo.lock by hand.)