Bring back part of PruneErasedDefs

[odersky]

Do make erased symbols private. This is needed so that we don't get false overriding relationships for effectively erased symbols.

Fixes #23451

[odersky]

Wojciech, can you check whether this fixes the failures in the open CB? Thanks!

[WojciechMazur]

Both geykyo/kyo and epfl-lara/lisa are now successfully building with these changes.

The crash in the backend for lichess-org/lila is also gone, however, there is some different error now:

Error:  Error while emitting lila/feed/Env
Error:  assertion failed: ClassBType.info not yet assigned: Llila/feed/Feed$;
Error:  one error found

I've retried it locally and the build passed this time - I believe there is some race condition under -Ybackend-parallelism that caused it, but it's unrelated to this PR

[odersky]

Yes, -Ybackend-parallelism fails intermittently for me as well. I am not sure we can trust it yet.