distros.conf: enable DISTRO/updates along with DISTRO-backports

enable the security updates repository while installing backports
because backports may have been built against squeeze+security and
picked up bumped dependencies

Signed-off-by: Andreas Beckmann <anbe@debian.org>
lib/db: ensure initialization in public methods

always call _find_all_packages() or _compute_package_states()
before accessing lazy initialized structures

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-r,dwke,lib/db: add explicit compute_package_states()

this call must happen whit CWD == master-directory

Signed-off-by: Andreas Beckmann <anbe@debian.org>