consider priorities only for downloadable pkgs in resolver

consider priorities only for downloadable pkgs in resolver

A package which can't be downloaded anymore is very likely dropped from
a release and can therefore no longer be 'standard' (or similar). We
therefore do not grant points for them anymore and demote them to
prio:extra instead which helps other packages breaking them away even if
they have a lower priority.

The testcase was initially created by Michael Vogt and just amended.