cmd/go: emit an error in 'go mod vendor' if any replacement is in the vendor directory

[bcmills]

There is significant overlap between replace directives and the vendor directory, and users sometimes conflate the two (#29079 (comment)).

Since go mod vendor removes and replaces the entire contents of the vendor directory, it will fail (or at least produce very confusing behavior) if a replace directive points into vendor. We should detect that condition explicitly and emit a useful error.

(CC @jayconrod)

[gopherbot]

Change https://golang.org/cl/156400 mentions this issue: emit an error in 'go mod vendor' if any replacement is in the vendor directory