Implicit `DefaultPickingPlugins` dependency

[musjj]

ui-navigation/src/systems.rs

Lines 359 to 361 in 3c876fc

	#[cfg(feature = "pointer_focus")]
	app.add_plugins(DefaultPickingPlugins)
	.add_systems(PostUpdate, enable_click_request);

Right now this plugin automatically inserts DefaultPickingPlugins, but this will cause a conflict if the user wants to control the insertion of the plugin.

For example, I have customized DefaultPickingPlugins like this:

DefaultPickingPlugins
    .build()
    .disable::<BevyUiBackend>()
    .add(MyOwnUiBackend)

A user might also prefer to manually insert the picking plugins:

picking_core::CorePlugin,
picking_core::InteractionPlugin,
input::InputPlugin,
bevy_picking_raycast::RaycastBackend,

This will prevent this plugin from functioning in this kind of workflows. Maybe an implicit dependency is better in this case? Not so sure.