Do not count updates when counting the number of packages in a sack

Updates in a sack are actually solvables in libsolv, so when we add support for
updateinfo we want to ignore these. Also, we can't assume that nsolvables does
not have holes, so the (nsolvables-2) logic wasn't correct either.
Add updateinfo support to hawkey, using the existing parser in libsolv
Add methods to get details about package updates