static getGathererList() {
    const fileList = [
      ...fs.readdirSync(path.join(moduleDir, './gather/gatherers')),
      ...fs.readdirSync(path.join(moduleDir, './gather/gatherers/seo')).map(f => `seo/${f}`),
      ...fs.readdirSync(path.join(moduleDir, './gather/gatherers/dobetterweb'))
          .map(f => `dobetterweb/${f}`),
    ];
    return fileList.filter(f => /\.js$/.test(f) && f !== 'gatherer.js').sort();
  }
Returns list of gatherer names for external querying.
@return {Array<string>}