code: func (w *Wrapper) importDirsUsed() []string {
  	used := []string{}
  	for _, imp := range w.ImportDirs {
  		for _, proto := range w.ProtoFiles {
  			if strings.HasPrefix(proto, imp) {
  				used = append(used, imp)
  				break
  			}
  		}
  	}
  	return used
  }
docs: // importDirsUsed returns the set of import directories that contain
  // entries in the set of proto files.