"go env GOMOD" gives this for the main module already
but it's useful to be able to query other modules.
Using {{.Dir}} does not work if the go.mod was auto-synthesized.

Change-Id: If4844571e9e429b541de0d40c36ff4c5743b2031
Reviewed-on: https://go-review.googlesource.com/125656Reviewed-by: Bryan C. Mills <bcmills@google.com>