Reintroduce the check that all source files for the used format are
actually included.

The component is no longer part of the filename we store. This is needed
to allow having the same file in multiple components at the same time.

The backreferences were causing problems when creating new objects via
the ORM.

This avoids problems if inactive keyrings in locations not writable by
the archive user are referenced.

This method can later decide which policy queue to use for the security
archive.

version_compare returns values less than, equal to or greater than zero.
This value is not always -1, 0 or 2(!) as the documentation claims.

Reference: http://bugs.debian.org/680891