build queues: keep packages pending in policy queues

build queues: keep packages pending in policy queues

manage-build-queues will now keep packages that are still pending in a
policy queue. This is important for the security archive where we
already want packages to be built, but not accept packages into the
public archive yet.

Packages will now also be removed from build queues right away if they
are no longer present in any suite or policy queue associated with the
build queue. Before the packages would stay in the build queue until
they expired and still be visible by buildds.