Submodules aren't checked out for git dependencies in extra-deps

[isomorpheme]

Similar story to #38 - it's this fetchgit call again: https://github.com/cdepillabout/stacklock2nix/blob/22676dfc45fa1c33899ba1da1a23665172a18ba7/nix/build-support/stacklock2nix/default.nix#L381-L386

An example where this breaks things is libjwt-typed, where the cbits subdir has a submodule with header files etc. that are needed.

I've checked that the issue is fixed when I add submodules = true to fetchgit, and I can open a PR for that, but I've noticed that evaluation can slow down if there's repos involved that have large submodules (e.g. tensorflow), so I'm not sure if that's the way to go.