Extend layered analysis and diff to packages

[davidcassany]

I think it would be really nice to be able to analyze the packages of the image by layers, as it is already possible with files. So we could list the packages installed in each layer. Probably by adding new layered differ, something like container-diff analyze <image> --type=layerapt or --type=layerrpm. I believe it could be done without modifying the current rootfs and layerfs extraction, for apt and rpm the database could be diffed with the previous layer on layers array (the previous layer with a database, layers without database could be just skipped).

I could try to make some contribution into that direction, but I wonder first if this makes sense to maintainers. Do you think it could be valuable?