For producing source tarballs associated with releases, we (and anyone who wants to produce them) need all source packages used in all image flavors included in a release. Collecting packages from any one flavor isn't enough since package lists naturally differ in images for different platforms (VM guest agents, drivers, platform-specific utils...).
One way to do that would be to incrementally collect package lists while building all flavors but since flavor definitions are readily machine-readable TOML files, there's a simpler approach: produce a virtual "mega-flavor" from all definitions that will be used for building a release and run a build with it to get a complete list of packages right away.