make spec parser stricter

Signed-off-by: Onur Özkan <work@onurozkan.dev>

kaptan-spec/tests/ui/flight_core_contract.rs

@@ -1,9 +1,8 @@
 use kaptan_spec::spec;
 
 #[spec("flight_core_contract.spec.md")]
-struct FlightCore;
+mod flight_core {}
 
 fn main() {
-    let size = core::mem::size_of::<FlightCore>();
-    assert_eq!(size, 0);
+    assert_eq!(core::mem::size_of::<()>(), 0);
 }