setPreOrderIndexIfUnset(module, index) {
		const mgm = this._getModuleGraphModule(module);
		if (mgm.preOrderIndex === null) {
			mgm.preOrderIndex = index;
			return true;
		}
		return false;
	}
@param {Module} module the module
@param {number} index the index of the module
@returns {boolean} true, if the index was set