async resolveId (importee, importer) {
                let result = await container.hooks.resolveId(importee, importer);

                if (typeof result === 'object') {
                    if (result.external) {
                        return null;
                    }

                    return result.id;
                }

                return result;
            }
@param {string} importee 
@param {string} importer
@return {Promise<RollupResolveId>}