code: func NewWithCredentialsSource(credsSrc auth.CredentialsSource) *Disco {
  	return &Disco{
  		hostCache: make(map[svchost.Hostname]*Host),
  		credsSrc:  credsSrc,
  		Transport: httpTransport,
  	}
  }
docs: // NewWithCredentialsSource returns a new discovery object initialized with
  // the given credentials source.