Currently, the cache file is quite big ~ 70MB and parsing it during
the packaging-data-fetch-downstream job gives frequent out-of-memory issues.

This file contains a lot of unused data (information for all packages available
in Debian). In order to keep only used data (i.e. for packages available in Apertis),
A new file packaging-cache.yaml is generated containing only data for cache. It
is a subset of the packaging.yaml and it is used only during the
packaging-data-fetch-downstream job to load cache without triggering the oom issue.

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>