Stub files for empty modules with submodules won't be generated

[konn]

Summary

Currently, stub files are not generated for modules without any function/class, even if it has submodules.
This prevents pyright and mypy to retrieve information of modules qualified with partially imported module.

Steps to reproduce

Consider the following library definition:

use pyo3::prelude::*;
use pyo3_stub_gen::{define_stub_info_gatherer, derive::*};

#[gen_stub_pyfunction(module = "empty_super_module.sub_mod")]
#[pyfunction(name = "greet")]
pub fn greet() {
    println!("Hello from sub_mod!")
}

#[pymodule]
fn main_mod(m: &Bound<PyModule>) -> PyResult<()> {
    sub_mod(m)?;
    Ok(())
}

fn sub_mod(parent: &Bound<PyModule>) -> PyResult<()> {
    let py = parent.py();
    let sub = PyModule::new_bound(py, "sub_mod")?;
    sub.add_function(wrap_pyfunction!(greet, &sub)?)?;
    parent.add_submodule(&sub)?;
    Ok(())
}

define_stub_info_gatherer!(stub_info);

/// Test of unit test for testing link problem
#[cfg(test)]
mod test {
    #[test]
    fn test() {
        assert_eq!(2 + 2, 4);
    }
}

Running appropriate gen_stub with this, pyo3-stub-gen generates the following stub file:

• python/empty_super_module/sub_mod.pyi:

  # This file is automatically generated by pyo3_stub_gen
  # ruff: noqa: E501, F401
  
  
  def greet() -> None:
      ...

Note that there is no python/empty_super_module/__init__.py. Consider the following test function:

import empty_super_module


def test_sum_mod():
    empty_super_module.sub_mod.greet()

Expected Behaviour

All of pytest, mypy, and pyright should work fine.

Actual Behaviour

Pytest passes, but mypy warns that Cannot find implementation or library stub for module named "empty_super_module" and pyright fails with the following error:

/path/to/tests/test_empty_super_module.py
  /path/to/tests/test_empty_super_module.py:5:24 - error: "sub_mod" is not a known attribute of module "empty_super_module" (reportAttributeAccessIssue)
1 error, 0 warnings, 0 informations 

Possible solutions

1. Taking submodules into account when generating stub files.
2. Unconditionally generating stub files even for empty modules.