Implement the first step toward Multi-Arch by setting up a Group

Implement the first step toward Multi-Arch by setting up a Group
infrastructor for packages.

APT is now aware of the fact that a package A in architecture X can't
satisfy a dependency on package A in architecture Y - to handle these
packages are now identified by name and architecture, so different
architectures of the same package are handled internally as completly
different packages. This is great for pinning, dependency checking and
in many other situations, but sometimes we need to know which archs are
available for a given package: Here Groups come to our rescue!