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/

(sapling split of 4cd89e0be8be1ce68a8f68273e88a66c1a0227f5)
sixninetynine/pex,pombredanne/pex,jamesbroadhead/pex,URXtech/pex,sixninetynine/pex,snyaggarwal/pex,mzdanieltest/pex,kwlzn/pex,pombredanne/pex,kwlzn/pex,pantsbuild/pex,snyaggarwal/pex,URXtech/pex,jamesbroadhead/pex,mzdanieltest/pex,rouge8/pex,rouge8/pex,pantsbuild/pex,jsirois/pex,jsirois/pex