How to use the umi-core/lib/getUserConfig.getConfigByConfigFile function in umi-core

if (force) {
      cleanConfigRequireCache(cwd);
    }

    let config = null;
    const relativeFile = file.replace(`${paths.cwd}/`, '');
    this.relativeFile = relativeFile;

    const onError = (e, file) => {
      const msg = `配置文件 "${file.replace(`${paths.cwd}/`, '')}" 解析出错，请检查语法。
\r\n${e.toString()}`;
      this.printError(msg);
      throw e;
    };

    config = getConfigByConfigFile(file, {
      defaultConfig,
      onError,
    });

    config = this.service.applyPlugins('_modifyConfig', {
      initialValue: config,
    });

    // Validate
    for (const plugin of this.plugins) {
      const { name, validate } = plugin;
      if (config[name] && validate) {
        try {
          plugin.validate.call({ cwd }, config[name]);
        } catch (e) {
          // 校验出错后要把值设到缓存的 config 里，确保 watch 判断时才能拿到正确的值