Refactored PackageConstraint in two ways:
 1) split it into a package name and a 'PackageProperty' to make
    the code a bit cleaner;
 2) changed PackageName to 'Qualified PackageName'.

Added a Binary instance for Qualifier in PackagePath.hs (needed
for PackageConstraint).

Added pretty-printing code for PackageConstraint.

For now, all the code that creates a PackageConstraint just sets
the qualifier to 'unqualified', so this commit will not change
the external behaviour of cabal-install.