We construct a PostBuildProjectStatus which has sets of package ids
covering various properties, including:
 - out of date
 - definitely up to date
 - probably up to date
 - invalid due to dep changes
 - invalid due to direct build failure

We also persistently keep track of the "probably up to date" set between
builds.

In this patch we just calculate and maintain these things but don't yet
use them except for debug output. The idea is it should be useful for
things like maintaining the .ghc.environment, deleting stale build dirs
or package registrations, maintaining exe wrapers/symlinks etc.