Allow modules to define if they are single/multi-module builds

At the moment the submodule resolver requires 'branch' field in .gitmodules to determine that submodule should be cloned within the parent/modules folder. In case that field is missing, it will introduce hidden side-effect that will cause the submodule to be cloned as a top-level git repository, e.g:

 {QT_CI_REPO_DIR}/submodule/.git

instead of the parent's module submodule folder

 {QT_CI_REPO_DIR}/parent/modules/submodule/.git

This causes breakages in citests that assume static git-repos folder structure and will fail to assert when attempting to clone submodule repository.

Ideally, we should not have dependency to 'branch' field, but rather allow parent module to declare whether its submodules should be declared as separate module or as submodule.