add distros.conf

new configuration file describing
* distros (e.g. lenny, squeeze, sid)
* partial distros (e.g. squeeze-security (needs squeeze) or
  experimental (needs sid)
* non-default mirrors (archived distros, backports)

Signed-off-by: Andreas Beckmann <anbe@debian.org>
lib/conf: add class DistroConfig

for reading distros.conf

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-m: use DistroConfig

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s: use DistroConfig

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-r: use DistroConfig

Signed-off-by: Andreas Beckmann <anbe@debian.org>
lib/conf: Config: drop get_sources_url()/get_packages_url()

this is now handled with more flexibility in DistroConfig

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