make spec parser stricter

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

kaptan-spec/tests/ui/fail_non_module_item.stderr

@@ -0,0 +1,7 @@
+error: `#[spec]` may only be applied to a module
+ --> tests/ui/fail_non_module_item.rs:3:1
+  |
+3 | #[spec("flight_core_contract.spec.md")]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |
+  = note: this error originates in the attribute macro `spec` (in Nightly builds, run with -Z macro-backtrace for more info)