_execHandler(handlerName, ev) {
    /* eslint-disable global-require */
    for (const item of this._plugins) {
      let plugin;
      if (item.name.match(/^[./]/)) {
        const pluginPath = path.resolve(item.name);
        plugin = require(pluginPath);
      } else {
        module.paths.push('./node_modules');
        plugin = require(item.name);
        module.paths.pop();
      }

      if (!plugin[handlerName]) continue;

      ev.data.option = item.option;
      plugin[handlerName](ev);
    }
  }
exec plugin handler.
@param {string} handlerName - handler name(e.g. onHandleCode)
@param {PluginEvent} ev - plugin event object.
@private