AptFacade.get_packages() should return one object for each version of the package

[Björn Tillenius]

AptFacade.get_packages() currently return one package object per package name. However the code that reports which packages are installed in available expects to get one object per package version. This means that at the moment we report only the candidate version, so we miss some updates and installed packages.

Creating a list of Version objects would be a quick fix. The callsites of get_packages() will have to be adapted a bit, but that's easy work. After that we can rethink this and see how we can adapt the package API to deal with Package objects directly.