go/types: avoid importer.Default
It uses a throwaway FileSet, so all position info is
wrong, and potentially misleading.
(Various other helpers in go/types testing also use a
throwaway FileSet, and should really accept it as a parameter.)
Fixes #71272
Change-Id: I9d899b987837b4041a299aad5ec266cb4f5d125c
Reviewed-on: https://go-review.googlesource.com/c/go/+/643777
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>