cmd/go: remove loaderstate dependency

cmd/go: remove loaderstate dependency

This change removes the dependency on the module loader state from the
`QueryMatchesMainModulesError.Error()` method.

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

Change-Id: I47241587a0bf9b578931628f35ed3b936a0cb04a
Reviewed-on: https://go-review.googlesource.com/c/go/+/714700
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>