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

[isomorpheme]

Similar story to #38 - it's this fetchgit call again:

stacklock2nix/nix/build-support/stacklock2nix/default.nix

Lines 381 to 386 in 22676df

	rawSrc = builtins.fetchGit {
	url = haskPkgLock.git;
	name = srcName;
	rev = haskPkgLock.commit;
	allRefs = true;
	};

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.