cmd/go: make 'go get <module>@none' idempotent

Before this change, 'go get <module>@none' for a module not in the build list
would add the module to go.mod (with the explicit version string "none").
Subsequent go commands would fail with 'invalid module version "none"'.

Change-Id: Iebcaeab89eb19959f0a9aeda836f179962953313
Reviewed-on: https://go-review.googlesource.com/127215


Reviewed-by: Russ Cox <rsc@golang.org>