Declarative modules: submodule declarations are not considered covered

[alex]

From the coverage report on https://github.com/pyca/cryptography/pull/11159

As you can see the mod ... lines are all considered uncovered. This possibly has something to do with the spans on the generated code.

[alex]

I suspect, but am not positive, that this is because the __pyo3_init functions are generated for submodules, but obviously never called.

[alex]

Ok, my idea here is:

• Add a new submodule parameter to pymodule: pymodule(submodule)
• Automatically add this attribute for nested modules (users could add it themselves for their own modules)
• When a module is marked submodule, we no longer emit __pyo3_init

WDYT?