cmd/go: test package patterns with multiple modules
cmd/go: test package patterns with multiple modules

This change replaces https://golang.org/cl/125835.

Updates #26317.

Change-Id: I38ae1f93e5f5c86737a4b489df498c18b179781d
Reviewed-on: https://go-review.googlesource.com/128637
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

akutz/go,golang/go,CAFxX/go,cloudflare/go,josharian/go,akutz/go,akutz/go,golang/go,nawawi/go,josharian/go,nawawi/go,cloudflare/go,CAFxX/go,josharian/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,akutz/go,golang/go,golang/go,golang/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,golang/go,akutz/go,CAFxX/go,nawawi/go,akutz/go,nawawi/go,nawawi/go,CAFxX/go,josharian/go,cloudflare/go,CAFxX/go,josharian/go,josharian/go,cloudflare/go,josharian/go,akutz/go,CAFxX/go,cloudflare/go,cloudflare/go,akutz/go,josharian/go,cloudflare/go