distros.conf: add virtual squeeze-proposed, wheezy-proposed

these are virtual full distros created by merging a full distro with some
partial distros

Signed-off-by: Andreas Beckmann <anbe@debian.org>
lib/db: add load_packages_urls() methods

Signed-off-by: Andreas Beckmann <anbe@debian.org>
lib/conf: add get_{packages,sources}_urls() stubs

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-[msr]: switch to load_packages_urls()

avoid code duplication and support merging several packages files

Signed-off-by: Andreas Beckmann <anbe@debian.org>
lib/conf: resolve candidates to multiple URLs

Signed-off-by: Andreas Beckmann <anbe@debian.org>
lib/db: drop obsolete read_packages_file()

this will break the unittest that still uses this ...

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p.conf.anbe: [wheezy-proposed] (wheezy + wheezy/updates + wheezy-proposed-updates)

* virtual full distro of the upcoming stable point release

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