cmd/go: use local loader state in test

cmd/go: use local loader state in test

This change converts the import test to use a local module loader
state variable, instead of the dependency on global state.

This commit is part of the overall effort to eliminate global
modloader state.

Change-Id: I5687090c160bf64ce36356768f7cf74333fab724
Reviewed-on: https://go-review.googlesource.com/c/go/+/711138
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>