make spec parser stricter

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

kaptan-spec/tests/ui/fail_missing_sidecar.rs

@@ -1,8 +1,7 @@
 use kaptan_spec::spec;
 
 #[spec("fail_missing_sidecar.spec.md")]
-struct MissingSpec;
+mod missing_spec {}
 
 fn main() {
-    let _ = core::mem::size_of::<MissingSpec>();
 }