code: function readDirs(dir, cb)  {
    // find top-level module dirs
    if (Array.isArray(dir)) {
      // dir is a list of paths
      readDirsParallel(dir, cb);
    } else {
      // dir is a single path value
      readDirAbs(dir, cb);
    }
  }
docs: Get the contents of one or more directories if dir is an array, do this in parrallel.