go/types: add errorcalls_test, apply it, and fix errorf call sites

go/types: add errorcalls_test, apply it, and fix errorf call sites

The errorcalls_test makes sure that we use error instead of errorf
where possible. Copied from types2 and adjusted for go/types.

Change-Id: Ib0572308c87e4415bf89aec8d64e662abe94754b
Reviewed-on: https://go-review.googlesource.com/c/go/+/441958
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>