check for redeclaration of package name

R=r
DELTA=10  (10 added, 0 deleted, 0 changed)
OCL=28585
CL=28591

test/import1.go

+// errchk $G -e $D/$F.go
+
+// Copyright 2009 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.
+
+// check for import conflicts
+
+package main
+
+import (
+	"bufio";
+	bufio "os";	// ERROR "redeclaration"
+)