Make fetchFromGitHub support submodules

[copumpkin]

Currently if your source bundle uses git submodules, you either need to use fetchgit or manually string together fetchFromGitHub invocations to plug them into the relevant locations.

The former already works, but is less efficient than it needs to be. The latter could be done by calling fetchFromGitHub recursively, parsing the .gitmodules file, and pulling out submodules as needed. The submodules could themselves be fetched with the equivalent of fetchgit (with non-GH submodules, assuming its builder script is reusable) or fetchFromGitHub (if the submodules come from GH)

[domenkozar]

There are a few packages using fetchgit for this reason, we should port them when this is done as a proof of concept

[Profpatsch]

The current fix uses fetchgit as Fallback, ideally we’d like to have a recursive fetchtar/fetchzip.

[matthewbauer]

I believe this can be closed now.