make various entity wrapper type modules public

[Victoronz]

Objective

Part of the #16547 series.

The entity wrapper types often have some associated types an aliases with them that cannot be re-exported into an outer module together.
Some helper types are best used with part of their path: bevy::ecs::entity::index_set::Slice as index_set::Slice.
This has already been done for entity::hash_set and entity::hash_map.

Solution

Publicize the index_set, index_map, unique_vec, unique_slice, and unique_array modules.

Migration Guide

Any mention or import of types in the affected modules have to add the respective module name to the import path.
F.e.:
bevy::ecs::entity::EntityIndexSet -> bevy::ecs::entity::index_set::EntityIndexSet

[bushrat011899]

Makes sense!