The importer.For function logically requires a FileSet, but did not
when it was first created because export data did not then record
position information. This change adds a new function, ForCompiler,
that has an additional FileSet parameter, and deprecates the For
function.

Before this change, cmd/vet would report confusing spurious
positions for token.Pos values produced by the importer.
The bug is essentially unfixable in cmd/vet.

This CL includes a test that the FileSet is correctly populated.

The changes to cmd/vendor will be applied upstream in a follow-up.

Fixes #28995

Change-Id: I9271bcb1f28e96845c913e15f0304bac93d4d4c4
Reviewed-on: https://go-review.googlesource.com/c/152258
Run-TryBot: Alan Donovan <adonovan@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Robert Griesemer <gri@golang.org>