Extract a BinaryPackageUniverse from the InstallabilityTester

The InstallabilityTester is suffering from a lack of clear purpose
because it serves multiple.  This commit extracts most of one of these
purposes into the BinaryPackageUniverse class while retaining the
original API of the InstallabilityTester.

Signed-off-by: Niels Thykier <niels@thykier.net>