Fix/recursive jpackage stubs

Addresses the case of stubs not existing for parent packages for the JPackage case.

For example, previously when generating stubs for java.util.zip with something like:

python -m stubgenj java.util.zip

The stubs at java-stubs/__init__.pyi would be empty, but in practice they need to include the java.__module_protocol__ implementation along the lines of:

import java.util
import typing


class __module_protocol__(typing.Protocol):
    # A module protocol which reflects the result of ``jp.JPackage("java")``.

    util: java.util.__module_protocol__

In this example, we would then need a similar stubfile for the java-stubs/util/__init__.pyi file, which would again provide a __module_protocol__ for the zip package.

What this change doesn't do is automatically "promote" stub __module__protocol__s for items not explicitly mentioned. The example above results in stubs being generated for a bunch of other modules (e.g. java.util.spi) but this __module_protocol__ is not added to java.util (unless explicitly asked for). I believe this is consistent with what we are trying to achieve whereby we only give suggestions for packages that are explicitly enabled (and all of their children recursively). I didn't add tests to this end, and believe it would be tricky with the current test infrastructure.