cmd/go/internal/module: fix validation for module paths ending with /v

Unlike "/v1", "/v" is not likely to be mistaken for a semantic import path.

Change-Id: I024647d78c79c7761b98ddeccdc7e259ca94b568
Reviewed-on: https://go-review.googlesource.com/c/152738


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>