cmd/go: properly call PackageModuleRoot to get modroot for index
cmd/go: properly call PackageModuleRoot to get modroot for index

PackageModuleRoot needs to be called with the package's path, not
its directory on disk.

Change-Id: I080fe8ce2aeb72e1466624db81595a00915606bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/410820
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Michael Matloob <4562438b12c14e81a47550acde616ff328a909f8@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Michael Matloob <4562438b12c14e81a47550acde616ff328a909f8@golang.org>

golang/go,nawawi/go,golang/go,nawawi/go,golang/go,golang/go,golang/go,nawawi/go,nawawi/go,nawawi/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,golang/go