Internal Entities

[Trashtalk217]

Objective

As we move more stuff to entities, it's a good idea to keep these entities quasi-private. We do not want to confuse users by having to explain everything as being an entity.

This came out of #19711.

Solution

This PR introduces the concept of internal entities, entities marked by the Internal component, that are filtered out by queries through DefaultQureyFilters and also don't show up for World::entity_count().

Testing

Added a test.

[Trashtalk217]

There are two options for the tests, and I would like some comments as to what would be better:

1. Replace every assert_eq!(world.entities().len(), 1) with assert_eq!(world.count_entities(), 1) which ignores internal entities.
2. Replace every assert_eq!(world.entities().len(), 1) with assert!(world.check_entities(1, |world| { // some block })) which just registers the change.

[chescock]

There are two options for the tests, and I would like some comments as to what would be better:

1. Replace every assert_eq!(world.entities().len(), 1) with assert_eq!(world.count_entities(), 1) which ignores internal entities.
2. Replace every assert_eq!(world.entities().len(), 1) with assert!(world.check_entities(1, |world| { // some block })) which just registers the change.

If we can, I think it would be better to rewrite those tests to stop using entities.len() and instead query the entities they care about. Like, the ones that spawn an entity with TableStored can do world.query::<&TableStored>().iter(&world).len(). The ones that do spawn_empty() will need some new marker component for the test.

(Copied from my reply on Discord.)

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Please review the instructions for writing migration guides, then expand or revise the content in the migration guides directory to reflect your changes.

[alice-i-cecile]

Generally on board but:

1. I have some simple doc suggestions.
2. I feel strongly about using queries in our tests rather than counting the total number of entities anywhere. I'll help yell at people too!

[alice-i-cecile]

Apparently this PR does not need a migration guide, but changes to make observers etc use this will.

[Trashtalk217]

I've thought about it some more, and I came to agree.

world.query<Component>().len()

would be wonderful to have, but since I'm not going to re-architect queries right now, I'll take the alternative.
I'll remove the entity count stuff from this PR and go through the tests - replacing every entity_count with a query - in a separate PR.

[chescock]

Hmm, I think there are some tricks that could make a query.count() method more efficient than query.iter().count(). That seems worth doing regardless of the outcome here, so let me push that up... #20230.