hide Translation-* in 'apt-cache policy' output

hide Translation-* in 'apt-cache policy' output

Translation-* files are internally handled as PackageFiles which isn't
super nice, but giving them their own struct is a bit overkill so let it
be for the moment. They always appeared in the policy output because of
this through and now that they are properly linked to a ReleaseFile they
even display all the pinning information on them, but they don't contain
any packages which could be pinned… No problem, but useless and
potentially confusing output.

Adding a 'NoPackages' flag which can be set on those files and be used
in applications seems like a simple way to fix this display issue.