cmd/go: disallow version string in go mod init module path

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>