Minimizes the differences with go/internal/gcimporter.Import.
Note that the copy in cmd/compile/internal/importer is currently
only used in tests.
The delta between the two Import functions is now just types vs types2.
Change-Id: I5e94d3aa5bbdb78252e47310c95807f63e27ef3d
Reviewed-on: https://go-review.googlesource.com/c/go/+/626698
Commit-Queue: Tim King <taking@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>