cmd/gc: Don't accept qualified names as literal keys

Fixes #4067.

R=golang-dev, minux.ma, dave, rsc
CC=golang-dev
https://golang.org/cl/6622056

test/fixedbugs/bug462.go

@@ -0,0 +1,19 @@
+// errorcheck
+
+// Copyright 2012 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 "os"
+
+type T struct {
+	File int
+}
+
+func main() {
+	_ = T {
+		os.File: 1, // ERROR "unknown T field"
+	}
+}