function isExternal (config, name) {
    if (config && config.external) {
        let external = config.external;
        if (Array.isArray(external)) {
            return external.indexOf(name) > -1;
        }

        if (typeof external === 'function') {
            return external(name, undefined, undefined);
        }
    }

    return false;
}
@param {RollupConfigContainer} config 
@param {string} name 
@return {boolean | void}