cmd/gc: make missing package error fatal

No longer continue assuming package main.

Fixes #4776.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/12677043

src/cmd/gc/go.y

@@ -136,8 +136,7 @@ package:
 	{
 		prevlineno = lineno;
 		yyerror("package statement must be first");
-		flusherrors();
-		mkpackage("main");
+		errorexit();
 	}
 |	LPACKAGE sym ';'
 	{

src/cmd/gc/y.tab.c

@@ -2428,8 +2428,7 @@ yyreduce:
     {
 		prevlineno = lineno;
 		yyerror("package statement must be first");
-		flusherrors();
-		mkpackage("main");
+		errorexit();
 	}
     break;
 

test/fixedbugs/issue4776.go

+// errorcheck
+
+// Copyright 2013 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.
+
+// Issue 4776: missing package declaration error should be fatal.
+
+type MyInt int32 // ERROR "package statement must be first"
+