go/types, types2: replace (internal) writePackage with packagePrefix

go/types, types2: replace (internal) writePackage with packagePrefix

This makes it easier to use the package string prefix in some cases
(cleanup).

Change-Id: I0ae74bf8770999110e7d6e49eac4e42962e78596
Reviewed-on: https://go-review.googlesource.com/c/go/+/451795
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>