ModuleNotFoundError: No module named 'supermodule.submodule'; 'supermodule' is not a package

🐛 Bug Reports

I am working for writing Python extension, but facing import submodule error.

ModuleNotFoundError: No module named 'supermodule.submodule'; 'supermodule' is not a package

What do I expected

Import submodule like usual Python package.

🌍 Environment

• Your operating system and version:
  ArchWSL version 19.11.16.0 on Windows Subsystem Linux 2 version Windows Insiders Slow ring 10.0.19041

$ uname -a
Linux xxxxxx 4.19.84-microsoft-standard #1 SMP Wed Nov 13 11:44:37 UTC 2019 x86_64 GNU/Linux

• Your python version:

$ python -V
Python 3.8.1

• How did you install python (e.g. apt or pyenv)? Did you use a virtualenv?:

$ pyenv virtualenv system xxxxxx

• Your rust version (rustc --version):

rustc 1.43.0-nightly (c9290dcee 2020-02-04)

• Are you using the latest pyo3 version? Have you tried using latest master (replace version = "0.x.y" with git = "https://github.com/PyO3/pyo3")?
  Yes.

💥 Reproducing

Please provide a minimal working example. This means both the rust code and the python.

Please also write what exact flags are required to reproduce your results.
src/lib.rs

use pyo3::prelude::*;
use pyo3::{wrap_pyfunction, wrap_pymodule};
use pyo3::types::IntoPyDict;

#[pyfunction]
fn subfunction() -> String {
    "Subfunction".to_string()
}

#[pymodule]
fn submodule(_py: Python, module: &PyModule) -> PyResult<()> {
    module.add_wrapped(wrap_pyfunction!(subfunction))?;
    Ok(())
}

#[pymodule]
fn supermodule(_py: Python, module: &PyModule) -> PyResult<()> {
    module.add_wrapped(wrap_pymodule!(submodule))?;
    Ok(())
}

Cargo.toml

[package]
name = "supermodule"
version = "0.1.0"
authors = ["Tang Ziya <tcztzy@gmail.com>"]
edition = "2018"

[package.metadata.maturin]
classifier = [
    "Development Status :: 3 - Alpha",
    "Programming Language :: Python",
    "Programming Language :: Rust",
]

[lib]
name = "supermodule"
crate-type = ["cdylib", "rlib"]

[features]
default = []

[dependencies]
pyo3 = { git = "https://github.com/PyO3/pyo3", features = ["extension-module"] }

[dev-dependencies]

$ maturin develop
$ python -c "from supermodule.submodule import subfunction"
ModuleNotFoundError: No module named 'supermodule.submodule'; 'supermodule' is not a package

Start with the reproducing example in src/lib.rs and its Cargo.toml, then run maturin develop followed by the reported Python import command. Compare the module layout produced by this setup with the expected import path; done means from supermodule.submodule import subfunction succeeds without the ModuleNotFoundError.