Get namespace packages (list) but only for
        single_version_externally_managed installations and empty otherwise.

def _get_SVEM_NSPs(self):
        """
        Get namespace packages (list) but only for
        single_version_externally_managed installations and empty otherwise.
        """
        # TODO: is it necessary to short-circuit here? i.e. what's the cost
        # if get_finalized_command is called even when namespace_packages is
        # False?
        if not self.distribution.namespace_packages:
            return []

        install_cmd = self.get_finalized_command('install')
        svem = install_cmd.single_version_externally_managed

        return self.distribution.namespace_packages if svem else []