code: async function filesInDirectories(dirPaths, options) {
    let files = await Promise.all(makeArray(dirPaths).map(dirPath=>_filesInDirectory(dirPath, options)));
    return flattenDeep(files);
  }
docs: Get all the files in a collection of directories.
  
  @param {Array|Set|string} dirPaths     Path(s) to get files from.
  @param [options]                       Import options object.