Check if the given path is a git submodule.

        See the docstrings for ``_check_submodule_using_git`` and
        ``_check_submodule_no_git`` for further details.

def _check_submodule(self):
        """
        Check if the given path is a git submodule.

        See the docstrings for ``_check_submodule_using_git`` and
        ``_check_submodule_no_git`` for further details.
        """

        if (self.path is None or
                (os.path.exists(self.path) and not os.path.isdir(self.path))):
            return False

        if self.use_git:
            return self._check_submodule_using_git()
        else:
            return self._check_submodule_no_git()