bevy_ui_picking_backend feature can't be disabled #19742

Bevy version

main (d2e1f72)

What you did

Disable default features of bevy and enable all default features but bevy_ui_picking_backend.

What went wrong

1. You also have to disable bevy_core_widgets because it unconditionally enables bevy_ui_picking_backend.
2. Then it fails to compile, as the focus module in bevy_ui uses the picking backend unconditionally:

   bevy/crates/bevy_ui/src/focus.rs

   Line 2 in d2e1f72

   picking_backend::clip_check_recursive, ui_transform::UiGlobalTransform, ComputedNode,