gc: fix import name resolution

Fixes #403.

R=ken2
https://golang.org/cl/180052

src/cmd/gc/go.y

@@ -195,7 +195,7 @@ import_stmt:
 		}
 		my->def = pack;
 		my->lastlineno = $1;
-		import->block = 1;	// at top level
+		my->block = 1;	// at top level
 	}
 
 

test/fixedbugs/bug233.go

+// $G $D/$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.
+
+package main
+import p "fmt"
+var _ = p.Print
+var fmt = 10