Using trait as _ to avoid namespace pollution

[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 #16163 (comment)

[benfrankel]

This is a duplicate of #13959 btw :)

Although this issue does seem to go further and say that use trait as _ should be preferred even if it's not being re-exported (not just pub use trait as _). I'd personally agree with that as well, although that's more of a question of style.