This moves the functions of the PhasedUpgrader class into
various other classes so they can be publicly exposed.

This introduces three new functions:

pkgDepCache::PhasingApplied() tells you whether phasing should
be applied to the package.

pkgProblemResolver::KeepPhasedUpdates() keeps back updates that
have phasing applied.

pkgCache::VerIterator::IsSecurityUpdate() determines whether this
version contains security fixes.