Use register_dynamic for merging

[ElliottjPierce]

Objective

I found a bug while working on #17871. When required components are registered, ones that are more specific (smaller inheritance depth) are preferred to others. So, if a ComponentA is already required, but it is registered as required again, it will be updated if and only if the new requirement has a smaller inheritance depth (is more specific). However, this logic was not reflected in merging RequriedComponentss together. Hence, for complicated requirement trees, the wrong initializer could be used.

Solution

Re-write merging to work by extending the collection via require_dynamic instead of blindly combining the inner storage.

Testing

I created this test to ensure this bug doesn't creep back in. This test fails on main, but passes on this branch.

    #[test]
    fn required_components_inheritance_depth_bias() {
        #[derive(Component, PartialEq, Eq, Clone, Copy, Debug)]
        struct MyRequired(bool);

        #[derive(Component, Default)]
        #[require(MyRequired(|| MyRequired(false)))]
        struct MiddleMan;

        #[derive(Component, Default)]
        #[require(MiddleMan)]
        struct ConflictingRequire;

        #[derive(Component, Default)]
        #[require(MyRequired(|| MyRequired(true)))]
        struct MyComponent;

        let mut world = World::new();
        let order_a = world
            .spawn((ConflictingRequire, MyComponent))
            .get::<MyRequired>()
            .cloned();
        let order_b = world
            .spawn((MyComponent, ConflictingRequire))
            .get::<MyRequired>()
            .cloned();

        assert_eq!(order_a, Some(MyRequired(true)));
        assert_eq!(order_b, Some(MyRequired(true)));
    }

Note that when the inheritance depth is 0 (Like if there were no middle man above), the order of the components in the bundle still matters.

Does this need a Migration Guide?

I don't believe this needs a migration guide since it's a bug fix, but it could break user's complex requirement tree if they depended on the bug. If I should add this, let me know.