Follow up about #[namespace = unquoted::path] syntax

[dtolnay]

This was proposed for Rust in rust-lang/rust#76734 but deferred until after rust-lang/rust#55414 (comment) / rust-lang/rust#77271. If those issues stall out, we should poke this again.

[dtolnay]

Reopened rust-lang/rust#76734.

[dtolnay]

Landed in rust-lang/rust#78837 unstable for rust 1.50.0. Tracking issue: rust-lang/rust#78835.