split the code in figure_out_kept:

split the code in figure_out_kept:
we need a somewhat complete updateset so we can proceed with handling
dependencies.

The case where the signature is equal is not an issue, but the case
where we need to keep the package because it still contains oldlibs is!

So we do that AFTER dependencies so that dependencies have been updated
and we possibly depend on old libs (which may happen even if the signature
didn't change)