go/types, types2: report "undefined: p.x" instead of "x not declared by package p"

go/types, types2: report "undefined: p.x" instead of "x not declared by package p"

This matches the compiler's long-standing behavior.

For #55326.

Change-Id: Icd946b031b1b6e65498fb52bceb4a53807732463
Reviewed-on: https://go-review.googlesource.com/c/go/+/432556
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>