Perform additional cheap integrity checks as part of `checkForUpdates`

[Herbert Valerio Riedel]

tbd

[gbaz]

If a user has an inconsistent local repo (missing files, truncated files) cabal may get confused. If we verified the integrity of the local repo as part of the check for updates, this could also choose to repair the local repo cache if it were broken. I think this would be an important ticket.