pluginoriginpip: use importlib instead of pkg_resources

[abderrahim]

No description provided.

[abderrahim]

It looks like we can use https://pypi.org/project/backports.entry-points-selectable/ to fix the failure on python 3.8.

[abderrahim]

Now that we dropped python 3.8 support, we don't need to worry about it.

[juergbi]

The Python package tests in CI are failing with:

       File "/tmp/venv/lib/python3.9/site-packages/buildstream/_pluginfactory/pluginoriginpip.py", line 35, in get_plugin_paths
        from packaging.requirements import Requirement, InvalidRequirement
      File "/tmp/venv/lib/python3.9/site-packages/pluginbase.py", line 438, in plugin_import
        return self._system_import(import_name, globals, locals,
    ModuleNotFoundError: No module named 'packaging'

And Debian 11 (Python 3.9) is also not happy:

  File "/home/testuser/buildstream/src/buildstream/_pluginfactory/pluginoriginpip.py", line 75, in get_plugin_paths
    detail="{} {} is installed but {} is required".format(dist.name, dist.version, package),
AttributeError: 'PathDistribution' object has no attribute 'name'

[juergbi]

There are still a few remaining references to pkg_resources in the repository. Can we replace them in the same branch or does it make sense to defer that to a follow-up?

[abderrahim]

CI is happy now.

• I used importlib_metadata for python 3.9 that doesn't have all the needed features in importlib.metadata.
• I moved setuptools to dev-requirements, as it's still used by tests. We can port tests as a followup.
• I added packaging to the requirements