cmd/go: make ImportMissingError work with local state
This change adds fields to the type `ImportMissingError` so
that the `Error()` method can be called without accessing the global
`LoaderState` variable.
This commit is part of the overall effort to eliminate global
modloader state.
Change-Id: Ib313faeb27ae44e9ac68086313633ce742f596dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/711121
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Matloob <matloob@golang.org> Reviewed-by: Michael Matloob <matloob@google.com>