Automatically treat nested modules as submodules

fixes PyO3#4286

newsfragments/4308.changed.md

@@ -0,0 +1 @@
+Nested declarative `#[pymodule]` are automatically treated as submodules (no `PyInit_` entrypoint is created)

pyo3-macros-backend/src/module.rs

@@ -237,6 +237,7 @@ pub fn pymodule_module_impl(
                     )? {
                         set_module_attribute(&mut item_mod.attrs, &full_name);
                     }
+                    item_mod.attrs.push(parse_quote!(#[pyo3(submodule)]));
                 }
             }
             Item::ForeignMod(item) => {

tests/test_declarative_module.rs

@@ -115,7 +115,7 @@ mod declarative_module {
         }
     }
 
-    #[pymodule(submodule)]
+    #[pymodule]
     #[pyo3(module = "custom_root")]
     mod inner_custom_root {
         use super::*;