Avoid deprecated load_module() in pkg_resources namespace delaration

[encukou]

Summary of changes

Fixes #2493

As I said in the issue,

As for the calling of load_module in pkg_resources.__init__, it seems to me that the only purpose of the call is to ensure the module is loaded, so that its __path__ can be fixed up using _rebuild_mod_path. The call uses the default loader.

So, it could be replaced by importlib.import_module(packageName). From the load_module docs, the only thing it does in addition to what import_module does is reloading the module if it already exists. To me, that seems like an unintended side effect.
But I could also be off; the docs are written for implementing of loaders, not for calling them.

Pull Request Checklist

• Changes have tests (existing related tests no longer warn under Python 3.10; warnings are treated as errors by default)
• News fragment added in changelog.d/.
  (See documentation for details)