Support for git sub modules in vendoring in tools.nix

[kolloch]

#76 introduced tools.nix and nix functions for generating the build files in derivations. That allows users to skip checking in the generated Cargo.nix.

This does not work for "sub modules" yet.

[evanjs]

Checking to make sure I'm on the right page, here:

In trying to build something using generatedCargoNix, I am getting
Checksum for interop_rust 0.5.1 (git+ssh://git@<private-git-repo>/leap/interop_rust.git#bd8f3cd3cd952abb2997de0e4a747e22d2bda8c7) not found in crate-hashes.json
where interop_rust is a submodule.

This is done with the currently unreleased submodule functionality from fetchGit (introduced with NixOS/nix#3166).

Is it likely that I am encountering this issue?
Unless I'm misunderstanding the scope of tools.nix, it seemed sort of odd to need crate-hashes.json if I'm trying to refer to a remote repository that doesn't have Cargo.nix or etc checked in.

I am mostly wondering if I am encountering either this, #102, or both.