lib/db: load_alternate_versions_from_packages_urls(): new method

to replace the Version of all packages loaded from an older distro
with the current version (or "None") in the final distro

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-m,p-r: use load_alternate_versions_from_packages_urls()

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p: ignore backup files from base-passwd

these backups are created by base-passwd and modified on upgrades that
change the shell for most system accounts to /usr/sbin/nologin (#274229)

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