code: func (intr *Interpreter) invokeLoader(key moduleKey) (dict starlark.StringDict, src string, err error) {
  	loader, ok := intr.Packages[key.pkg]
  	if !ok {
  		return nil, "", ErrNoPackage
  	}
  	return loader(key.path)
  }
docs: // invokeLoader loads the module via the loader associated with the package.