runtime: don't say "different packages" if they may not be different

Fix the panic message produced for an interface conversion error to
only say "types from different packages" if they are definitely from
different packges. If they may be from the same package, say "types
from different scopes."

Updates #18911
Fixes #26094

Change-Id: I0cea50ba31007d88e70c067b4680009ede69bab9
Reviewed-on: https://go-review.googlesource.com/123395
Reviewed-by: Austin Clements <austin@google.com>

test/fixedbugs/issue26094.go

@@ -0,0 +1,49 @@
+// run
+
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "strings"
+
+var X interface{}
+
+type T struct{}
+
+func scopes() {
+	p, ok := recover().(error)
+	if ok && strings.Contains(p.Error(), "different scopes") {
+		return
+	}
+	panic(p)
+}
+
+func F1() {
+	type T struct{}
+	X = T{}
+}
+
+func F2() {
+	type T struct{}
+	defer scopes()
+	_ = X.(T)
+}
+
+func F3() {
+	defer scopes()
+	_ = X.(T)
+}
+
+func F4() {
+	X = T{}
+}
+
+func main() {
+	F1() // set X to F1's T
+	F2() // check that X is not F2's T
+	F3() // check that X is not package T
+	F4() // set X to package T
+	F2() // check that X is not F2's T
+}