cmd/go: add modload.State parameter to AllowedFunc

cmd/go: add modload.State parameter to AllowedFunc

This change makes the following functions methods on the State:
  * CheckAllowed
  * CheckExclusions
  * CheckRetractions

Doing so allows us to reduce the use of the global state variable in
downstream function calls.

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

Change-Id: I97147311d9de16ecac8c122c2b6bdde94bad9d8f
Reviewed-on: https://go-review.googlesource.com/c/go/+/711119
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>