cmd/go: fix error messages for go mod download in GOPATH mode

Checks if modules are enabled in GOPATH mode.
Error message returned when no version is provided. Relevant tests
updated. Test for GO111MODULE=off added.

Fixes #27783

Change-Id: I12cdaced5fa38a9c49c0ecfed4c479eb86ed061f
Reviewed-on: https://go-review.googlesource.com/c/go/+/179998


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>