dh_setup_copyright: Strip trailing '/@/...' from Rust CUs (and some perf fixes)

While debugging this I got annoyed by some seemingly random slow performance and thus snuck in a quick fix for that.

mentioned in issue infrastructure/apertis-issues#456 (closed)

requested review from @daissi

assigned to @refi64

merged

mentioned in merge request !26 (closed)

mentioned in merge request !24 (closed)