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/24862Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/reflect/all_test.go

@@ -2261,6 +2261,8 @@ func TestImportPath(t *testing.T) {
 		{TypeOf((*int64)(nil)), ""},
 		{TypeOf(map[string]int{}), ""},
 		{TypeOf((*error)(nil)).Elem(), ""},
+		{TypeOf((*Point)(nil)), ""},
+		{TypeOf((*Point)(nil)).Elem(), "reflect_test"},
 	}
 	for _, test := range tests {
 		if path := test.t.PkgPath(); path != test.path {

src/reflect/type.go

@@ -876,6 +876,9 @@ func (t *rtype) MethodByName(name string) (m Method, ok bool) {
 }
 
 func (t *rtype) PkgPath() string {
+	if t.tflag&tflagNamed == 0 {
+		return ""
+	}
 	ut := t.uncommon()
 	if ut == nil {
 		return ""