Fix naming overlap between `bevy_scene` and `bevy_scene_2`

[alice-i-cecile]

We cannot immediately remove bevy_scene, as the ECS world serialization pattern is not covered by bevy_scene_2 currently. However, we should give the nice names to the new BSN work.

Steps:

1. Rename bevy_scene to bevy_ecs_serialization.
2. Rename bevy_scene2 to bevy_scene.
3. Rename bevy_ecs_serialization::Scene and friends to resolve naming overlap. Alternatively, remove it from the prelude.
4. Make sure that the new bevy_scene prelude is re-exported in bevy::prelude.

Each of these should be a distinct PR for reviewability and merge conflicts.

Step 2 is going to break a lot of open PRs, but such is life.