Store Resources as components on singleton entities

[Trashtalk217]

This is part of #19731.

Resources as Components

Motivation

More things should be entities. This simplifies the API, the lower-level implementation and the tools we have for entities and components can be used for other things in the engine. In particular, for resources, it is really handy to have observers, which we currently don't have. See #20821 under 1A, for a more specific use.

Current Work

This removes the resources field from the world storage and instead store the resources on singleton entities. For easy lookup, we add a HashMap<ComponentId, Entity> to World, in order to quickly find the singleton entity where the resource is stored.

Because we store resources on entities, we derive Component alongside Resource, this means that

#[derive(Resource)]
struct Foo;

turns into

#[derive(Resource, Component)]
struct Foo;

This was also done for reflections, meaning that

#[derive(Resource, Reflect)]
#[refect(Resource)]
struct Bar;

becomes

#[derive(Resource, Component, Reflect)]
#[refect(Resource, Component)]
struct Bar;

In order to distinguish resource entities, they are tagged with the IsResource component. Additionally, to ensure that they aren't queried by accident, they are also tagged as being internal entities, which means that they don't show up in queries by default.

Drawbacks

• Currently you can't have a struct that is both a Resource and a Component, because Resource expands to also implement Component, this means that this throws a compiler error as it's implemented twice.
• Because every reflected Resource must also implement ReflectComponent you need to import bevy_ecs::reflect::ReflectComponent every time you use #[reflect(Resource)]. This is kind of unintuitive.

Future Work

• Simplify Access in the ECS, to only deal with components (and not components and resources).
• Newtype Res<Resource> to Single<Ref<Resource>> (or something similair).
• Eliminate ReflectResource.
• Take stabs at simplifying the public facing API.

[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-20934

If it's expected, please add the M-Deliberate-Rendering-Change label.

If this change seems unrelated to your PR, you can consider updating your PR to target the latest main branch, either by rebasing or merging main into it.

[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-20934

If it's expected, please add the M-Deliberate-Rendering-Change label.

If this change seems unrelated to your PR, you can consider updating your PR to target the latest main branch, either by rebasing or merging main into it.

[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-20934

If it's expected, please add the M-Deliberate-Rendering-Change label.

If this change seems unrelated to your PR, you can consider updating your PR to target the latest main branch, either by rebasing or merging main into it.