cmd/go: using strings.CutPrefix replace strings.HasPrefix and strings.TrimPrefix

cmd/go: using strings.CutPrefix replace strings.HasPrefix and strings.TrimPrefix

Change-Id: I143d05c24a3e897d0f3ee78dd16954c32ceae091
Reviewed-on: https://go-review.googlesource.com/c/go/+/435137
Run-TryBot: xie cui <523516579@qq.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>