cmd/go: use local state for telemetry

cmd/go: use local state for telemetry

This change adds a new loader state to satisfy the requirements for
the (currently unused) telemetry stats for the go command.

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

Change-Id: I6d4e38c91e5413d7649dfc6301e3ba35ee36c9b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/711136
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>