This uses git submodules. When cloning, the submodules' content will not be downloaded. You need
to run git submodule init then git submodule update. Or you can just add this option to the git
clone command --recurse-submodules.
If you are in a different branch, and add a submodule there, when you go back to main, the
submodule will not be there. You will need to do the checkout with --recurse-submodules.
Another way of handling this issues with submodules is to tell git to always use
--recurse-submodules. You can do that with the config git config submodule.recurse true. This
will work for every command that accepts a --recurse-submodules, except the clone command.