cmd/go: mod vendor: emit error if any replacement in vendor directory

cmd/go: mod vendor: emit error if any replacement in vendor directory

Fixes #29169

Change-Id: I3a0fd506c45ad999a2fc6f75f9b3e8a5118ad91b
Reviewed-on: https://go-review.googlesource.com/c/go/+/156400
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>