code: func (key moduleKey) String() string {
  	pkg := ""
  	if key.pkg != MainPkg {
  		pkg = "@" + key.pkg
  	}
  	return pkg + "//" + key.path
  }
docs: // String returns a fully-qualified module name to use in error messages.
  //
  // If is either "@pkg//path" or just "//path" if pkg is "__main__". We omit the
  // name of the top-level package with user-supplied code ("__main__") to avoid
  // confusing users who are oblivious of packages.