cmd/go: make Reset work with any State instance

cmd/go: make Reset work with any State instance

This commit updates the Reset function to work with any state
object in preparation for converting it to a method.

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

Change-Id: I6103842ea0a0528698217930afc0e34a2aa21eea
Reviewed-on: https://go-review.googlesource.com/c/go/+/712701
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>