gc: fix init of packages named main

This change removes the special case which existed
for handling the initalization of the main package,
so that other modules named 'main' get properly
initialized when imported.

Note that gotest of main packages will break in most
cases without this.

R=rsc
CC=golang-dev
https://golang.org/cl/4190050

test/init.go

@@ -0,0 +1,18 @@
+// errchk $G -e $D/$F.go
+
+// Copyright 2011 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 "runtime"
+
+func init() {
+}
+
+func main() {
+	init()         // ERROR "undefined: init"
+	runtime.init() // ERROR "unexported.*runtime\.init"
+	var _ = init   // ERROR "undefined: init"
+}