Fix cmmnbuild-dep-manager stub generation, and add appropriate tests.

Adds a new _site_packages method on Manager (it could have been moved out of Manager, but I kept it there for consistency with _dir, _user_dir and _dist_dir.

Makes stubgen an optional package pip install cmmnbuild-dep-manager[stubgen]

I looked at enabling subgen by default if installed, but was concerned with what the implications would be given that it starts the JVM (prevents Manager('pyjapc') type resolve calls from having control of JVM startup). To be investigated further.

Fixes the breakage as a result of merging the move from Manager._registered_packages to Manager._registered_modules.

Adds CI testing for the stubgen integration (JPype 1.2 only)