To prevent confusion, go mod init should not allow version strings in
the module path when provided as an argument. Instead, fail with a
useful error message.

Fixes #28803

Change-Id: I59272a91b042e32cef33c2e2116f760ca1def218
Reviewed-on: https://go-review.googlesource.com/c/150018
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>