test: add test for confusion with dot imports

The gccgo compiler would fail this test.  The fix for gccgo is
https://golang.org/cl/116960043 .

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/118000043

test/fixedbugs/bug488.dir/a.go

+// Copyright 2014 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 a
+
+var p2 = Printf		// ERROR "undefined"

test/fixedbugs/bug488.dir/b.go

+// Copyright 2014 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 a
+
+import . "fmt"
+
+var p1 = Print

test/fixedbugs/bug488.go

+// errorcheckdir
+
+// Copyright 2014 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.
+
+// The gccgo compiler had a bug: if one file in a package did a dot
+// import, then an earlier file in the package would incorrectly
+// resolve to the imported names rather than reporting undefined
+// errors.
+
+package ignored