Extension traits are re-exported by name

[benfrankel]

Extension traits can be re-exported without bringing their name into scope using underscore imports:

pub mod prelude {
    // This brings the trait AND its name into scope. This is how bevy currently does it.
    //pub use crate::commands::BuildChildrenTransformExt;

    // This brings the trait into scope.
    pub use crate::commands::BuildChildrenTransformExt as _;
}

Users can still pull in the name if needed:

use bevy_transform::prelude::*;
use bevy_transform::commands::BuildChildrenTransformExt;

Benefits:

• Clearly mark certain re-exports as extension traits
• Reduce name pollution (not that extension traits have high-value names, but still)

[BenjaminBrienen]

This is really interesting, I hadn't considered using trait as _ to avoid name collisions. At a glance this also works in preludes. For a future PR, could be nice to do this for extension style traits (e.g., AppAssetExt etc.) so the preludes aren't so heavy with names.

_Originally posted by @bushrat011899 in https://github.com/bevyengine/bevy/pull/16163#discussion_r1821705357_

[BenjaminBrienen]

I agree that use Trait as _ should be the standard in our code. I say even for normal use, not just exports.