Crate features: `debug` depends on `backend_bevy_ui` being enabled

[zhaop]

When I run:

cargo run --example minimal --no-default-features --features="debug"

I expected the debug text to appear whenever my mouse hovers the app window; instead, nothing appears.

Instead, I have to do:

cargo run --example minimal --no-default-features --features "debug,backend_bevy_ui"

Then the debug text appears.

---

Also, it seems to be specific to backend_bevy_ui; when I enable only the features it includes, but not backend_bevy_ui itself, the debug text still doesn't appear:

cargo run --example minimal --no-default-features --features "debug,bevy_picking_ui,bevy_ui,bevy_color"