Add an index of just the most recent version of each package

Currently packages/index.tar.gz has every version of every .cabal file ever. It's currently > 100Mb when uncompressed. Some clients are only interested in the most version of each package, so having a latest.tar.gz containing only the latest versions of each package might be useful. Certainly Hoogle would switch, resulting in > 200Mb disk space saving on each machine generating Hoogle databases.