Resources as Components Tracking Issue

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This issue does not exist to argue the merits of Resources as Components, for some of the rationale we refer to this HackMD (this really should've been an RFC, huh?). This issue exists to track the progress on implementation and to reference in code when it is only partially working. If there are steps you feel like should be added, or if there are problems with this approach, please let me know.

Implementation Plan

This is a tracking issue for the Resources as Components. This outline is roughly copied from #17485.

• Introduce Internal Entities, entities that are difficult to query, with the intend to make resource entities internal (Internal Entities #20204).
• Test robustness, to be less reliant on specific entity counts (Improve test robustness #20248, Adjust benches to be more robust #20207) . We deprecate world.iter_entities() and world._iter_entities_mut() as part of that effort Deprecate iter_entities and iter_entities_mut #20260.
• Store Resource behind a ResourceComponent, removing world.storages.resources Store resources as components on singleton entities (v2) #21346.
• (re-)Enable reflect auto-registration for ResourceComponent<SomeResource>.
• Remove clear_entities in favor of clear_all.
• Remove all of the resource internals (before this, they're marked with expect(dead_code)).
• Look into a #[maybe_entities] tag for MapEntities so you can implement it for generic types.
• Remove or shrink ReflectResource.
• Eliminate resources from Access.
• Remove resources from bevy_scene.
• Start removing non-send resources (PLAN STILL NEEDED).