go/types: consistently use _ prefix for unexported names that are exported in types2

go/types: consistently use _ prefix for unexported names that are exported in types2

Change-Id: Ic9b24b4b3a6336782023c7db40cc937f2dc743df
Reviewed-on: https://go-review.googlesource.com/c/go/+/461606
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>