special case check for this situation

	; cat >http.go
	package main
	import "http"  // intended the library, not this file
	^D
	; 6g http.go
	; 6g http.go
	http.go:4: export/package mismatch: init
	;

new error:

	http.6:7 http.go:3: cannot import package main

R=ken
OCL=27053
CL=27053

src/cmd/gc/go.y

@@ -173,6 +173,9 @@ import_package:
 	{
 		pkgimportname = $2;
 
+		if(strcmp($2->name, "main") == 0)
+			yyerror("cannot import package main");
+
 		// if we are not remapping the package name
 		// then the imported package name is LPACK
 		if(pkgmyname == S)