获取默认构建配置文件

@param {string} appType the type of mini application
@return {Object} get default config from 'lib/config/swan'
function getDefaultBuildConfig(appType) {
    if (appType === 'base') {
        throw new Error('illegal app type', appType);
    }

    let defaultConfPath = path.join(
        __dirname, '..', 'config', `${appType}.js`
    );
    if (fileUtil.isFileExists(defaultConfPath)) {
        return require(defaultConfPath);
    }
    else {
        return require('../config/base');
    }
}