Declarative modules don't set item's module properly

[wyfo]

Using this code

use pyo3::prelude::*;

#[pymodule]
mod my_extension {
    #[pymodule]
    mod submodule {
        #[pyfunction]
        fn foo() {}
    }
}

my_extension.submodule.foo.__module__ returns submodule instead of my_extension.submodule, as it would be expected for pure Python modules.

Relates to #3900

[davidhewitt]

Agreed this could be much better. There's additional runtime complexity because my_extension might itself be a submodule to a pure python root e.g. my_package.my_native_code_extension.submodule