Core: Move GER EntranceLookup onto ERPlacementState. Improve usefulness of on_connect

[BadMagic100]

What is this fixing or adding?

This makes a variety of general enhancements to the data available to the on_connect callback in GER. The motivator for this is to allow making additional entrance connections in this callback, which was always intended but implemented incredibly poorly

1. The EntranceLookup used by GER is now owned by and visible on the placement state. This allows it to be accessed from the on_connect callback, and allows callers to use er_state.connect without having to manage the EntranceLookup.
2. Added a find_target method on EntranceLookup, allowing callers to search for unplaced entrances to be able to make their own placements (functional equivalent of get_entrance for the uncached ER targets)
3. Updated the signature of on_connect to take in the paired targets as well so they can be used to make decisions about the side effects to be performed. This is technically breaking but tmk no merged worlds use GER yet so this should still be safe, but I believe it necessary even if it weren't safe and I don't see a way to avoid the break
4. Allow on_connect to signal that it has made entrance placements by returning True, and allow an additional state sweep afterwards if needed

How was this tested?

Added new unit tests for the new method and verified existing behavior has not regressed.