path: use bytealg.LastIndexByteString

path: use bytealg.LastIndexByteString

While strings.LastIndex{,Byte} cannot be used in package path, the
respective internal/bytealg function can be used.

Change-Id: If0ecc36484308221f50875c8609913f6f2887fba
Reviewed-on: https://go-review.googlesource.com/c/go/+/527855
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>