code: func ListPackageRefs(c context.Context, pkg string) (out []*Ref, err error) {
  	q := datastore.NewQuery("PackageRef").
  		Ancestor(PackageKey(c, pkg)).
  		Order("-modified_ts")
  	if err := datastore.GetAll(c, q, &out); err != nil {
  		return nil, errors.Annotate(err, "datastore query failed").Tag(transient.Tag).Err()
  	}
  	return
  }
docs: // ListPackageRefs returns all refs in a package, most recently modified first.
  //
  // Returns an empty list if there's no such package at all.