cmd/gc: change compile error to "use of package %S without selector"

At present, when a package identifier is used outside of a selector expression, gc gives the error "use of package %S outside selector". However, in the selector expression x.f, the spec defines f as the selector. This change makes the error clearer.

Fixes #7133.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/50060047

src/cmd/gc/typecheck.c

@@ -368,7 +368,7 @@ reswitch:
 		goto ret;
 
 	case OPACK:
-		yyerror("use of package %S not in selector", n->sym);
+		yyerror("use of package %S without selector", n->sym);
 		goto error;
 
 	case ODDD: