use #exists to provide good errors for common missing libraries

src/check_expr.cpp

@@ -8330,6 +8330,7 @@ gb_internal ExprKind check_basic_directive_expr(CheckerContext *c, Operand *o, A
 		    name == "assert" ||
 		    name == "defined" ||
 		    name == "config" ||
+			name == "exists" ||
 		    name == "load" ||
 		    name == "load_hash" ||
 		    name == "load_directory" ||