Implement union_data

This is the last package we need to add union_data to. It's a little more complicated because we don't currently have _tmp folders, so it may be a bigger change than the other packages' updates

Start by comparing the other packages' union_data updates, then inspect this package to understand how its lack of _tmp folders affects the change. The work is done when this package implements union_data consistently with those related updates and its package checks pass.

Written by the indexing model from the issue text.