Condense bevy_picking (input focus) feature into bevy_ui_picking feature

[alice-i-cecile]

Objective

The feature flags "bevy_picking" and "picking" have the same description despite fulfilling different goals. This is very confusing!

Fixes #22931.

The status quo before this PR.

• picking is a feature collection in bevy, which enables ["bevy_picking", "mesh_picking", "sprite_picking", "ui_picking"].
• the ui feature collection enables picking itself
• bevy_picking is defined in bevy, which defers to ["bevy_internal/bevy_picking"]
• in bevy_internal, this toggles bevy_picking = ["dep:bevy_picking", "bevy_input_focus?/bevy_picking"]
• this then toggles the bevy_picking dependency (apparently that works as if it were a feature?) in bevy_input_focus
• which is ultimately used to enable mouse / virtual cursor based input focus selection
• @viridia is concerned about making this mandatory because of worries about observer overhead

Solution

Condense bevy_picking at the bevy_internal level into bevy_ui_picking. There's no way someone wants general UI picking but not input focus picking. This fixes the redundancy, and groups functionality more reasonably, while still making it possible to avoid UI picking as a whole off if you're concerned for whatever reason.

Users who want even more granularity than that can of course compose the various Bevy crates however they please.

Alternative

1. Keep this feature around, renaming it to input_focus_picking.
2. Enable that feature inside of ui_picking.

This adds another niche feature flag at the top level, but makes it easier for users who want to use bevy_input_focus without bevy_ui. I have never met such a user, but maybe they exist!

[github-actions]

You added a new example but didn't add metadata for it. Please update the root Cargo.toml file.

[Smerom]

Looks like a good clarification and reasonable approach to me.

[ickshonpe]

Yeah I've only had a quick look at this but there already two approvals and this makes sense to me. Not sure about the observer overhead concerns. If we could add some concrete benchmarks in a follow up, like a a many_ui_observers or something, that should help.