cmd/go: report a helpful error when there are no modules in workspace

The current error message that no go.mod files were found is not
helpful, especially when a go.mod file exists in the current directory.

Fixes #49594

Change-Id: I750475ce8654eeb3e0a2857d5a2de1a9c6ede415
Reviewed-on: https://go-review.googlesource.com/c/go/+/365319


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>