ignore git submodules from checks

[hansvancalster]

I sometimes use git submodule to get the contents of an external repo in the local repo. Currently, checklist::check_project will check the submodule as well, while this is not needed.

When adding a submodule, git will write a file .gitmodules to the git-root of the project which lists all submodules. Here is an example of the contents:

[submodule "NCA_validatingextend"]
	path = NCA_validatingextend
	url = https://github.com/RCinbo/NCA_validatingextend

I think, by default, all paths that are listed in that file should be ignored by checklist::check_project (maybe also checklist::check_package, but I think submodules are not often or never used/needed in that case).