reflect: an unnamed type has no PkgPath

The reflect package was returning a non-empty PkgPath for an unnamed
type with methods, such as a type whose methods have a pointer
receiver.

Fixes #16328.

Change-Id: I733e93981ebb5c5c108ef9b03bf5494930b93cf3
Reviewed-on: https://go-review.googlesource.com/24862


Reviewed-by: David Crawshaw <crawshaw@golang.org>