Operation to inline default_visibility

[dimo414]

I'd like to lock down some of my BUILD files' default_visibility settings which are needlessly broad. I was hoping there's a way to do so with buildozer but I don't see an option for this. I would expect:

1. default_visibilty should be removed, rendering the package private by default
2. rules with explicit visibility declarations should be unmodified
3. remaining rules (except for export_files) should have their visibility set to the prior default