Add __hash__ to Link so that Packages correctly hash in resolve
Add __hash__ to Link so that Packages correctly hash in resolve

Reviewed at https://rbcommons.com/s/twitter/r/247/

VaybhavSharma/commons,abel-von/commons,nkhuyu/commons,nkhuyu/commons,nkhuyu/commons,brutkin/commons,Yasumoto/commons,pombredanne/commons,brutkin/commons,VaybhavSharma/commons,atollena/commons,Yasumoto/commons,brutkin/commons,brutkin/commons,atollena/commons,ericzundel/commons,ericzundel/commons,WCCCEDU/twitter-commons,jsirois/commons,cevaris/commons,ericzundel/commons,ericzundel/commons,Yasumoto/commons,Yasumoto/commons,atollena/commons,WCCCEDU/twitter-commons,ericzundel/commons,pombredanne/commons,brutkin/commons,VaybhavSharma/commons,pombredanne/commons,WCCCEDU/twitter-commons,cevaris/commons,abel-von/commons,abel-von/commons,nkhuyu/commons,nkhuyu/commons,cevaris/commons,pombredanne/commons,Yasumoto/commons,VaybhavSharma/commons,cevaris/commons,atollena/commons,cevaris/commons,VaybhavSharma/commons,abel-von/commons,pombredanne/commons,jsirois/commons,abel-von/commons,jsirois/commons,atollena/commons,jsirois/commons,jsirois/commons,WCCCEDU/twitter-commons,WCCCEDU/twitter-commons