Have st2 pack install checkout git submodules when cloning a pack repo

When a Pack is installed on Stackstorm using pack install - stackstorm performs a clone on the Repo (Code) - however this results in any submodules present in the repo not being checked out and only being an empty directory.

It would be useful if the clone_repo function also checked out any submodules that are present in the repo, as it would be helpful to add dependencies maintained in seperate repos to the pack as a submodule, while still being maintained externally.

In my use case a series of ansible playbooks are maintained in a seperate repo and I would like to also make them available as a pack to st2, and as I see it the best way to do that is via git submodules.