Fix an error when loading namespace packages

[ale-rt]

Fixes #25

Refs. plone/Products.CMFPlone#4126

[mister-roboto]

@ale-rt thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[mauritsvanrees]

This needs an extra import importlib.metadata. The current import importlib is not enough.

On Python 3.9, entry_points does not accept any arguments, so this needs some fixes, see the Compatibility note at the end of the entry points documentation. Maybe indeed use importlib_metadata as an extra requirement on Python 3.9, if that helps, in a similar way to your follow-up PR #29, though I don't like the extra dependency.

Also on other Python versions, the integration tests fail, because not all expected zcml is loaded. For example:

Failure in test test_configure (example.zopeintegration.tests.TestIntegration.test_configure)
Traceback (most recent call last):
  File "/usr/local/Cellar/[email protected]/3.12.8/Frameworks/Python.framework/Versions/3.12/lib/python3.12/unittest/case.py", line 58, in testPartExecutor
    yield
  File "/usr/local/Cellar/[email protected]/3.12.8/Frameworks/Python.framework/Versions/3.12/lib/python3.12/unittest/case.py", line 634, in run
    self._callTestMethod(testMethod)
  File "/usr/local/Cellar/[email protected]/3.12.8/Frameworks/Python.framework/Versions/3.12/lib/python3.12/unittest/case.py", line 589, in _callTestMethod
    if method() is not None:
  File "/Users/maurits/community/plone-coredev/6.2/src/plone.autoinclude/test-packages/example.basetestpackage/src/example/basetestpackage/integration_base.py", line 141, in test_configure
    self.files_have_been_loaded(self.configure_files)
  File "/Users/maurits/community/plone-coredev/6.2/src/plone.autoinclude/test-packages/example.basetestpackage/src/example/basetestpackage/integration_base.py", line 108, in files_have_been_loaded
    self.assertIn(package_context.path(filename), self.context._seen_files)
  File "/usr/local/Cellar/[email protected]/3.12.8/Frameworks/Python.framework/Versions/3.12/lib/python3.12/unittest/case.py", line 1152, in assertIn
    self.fail(self._formatMessage(msg, standardMsg))
  File "/usr/local/Cellar/[email protected]/3.12.8/Frameworks/Python.framework/Versions/3.12/lib/python3.12/unittest/case.py", line 715, in fail
    raise self.failureException(msg)
AssertionError: '/Users/maurits/community/plone-coredev/6.2/src/plone.autoinclude/test-packages/example.zopeaddon/src/example/zopeaddon/configure.zcml' not found in {'/Users/maurits/community/plone-coredev/6.2/src/plone.autoinclude/test-packages/example.zopeintegration/src/example/zopeintegration/meta.zcml', '/Users/maurits/community/plone-coredev/6.2/src/plone.autoinclude/src/plone/autoinclude/meta.zcml', '/Users/maurits/community/plone-coredev/6.2/src/plone.autoinclude/test-packages/example.different2/src/example/somethingelse2/configure.zcml', '/Users/maurits/community/plone-coredev/6.2/src/plone.autoinclude/test-packages/example.zopeintegration/src/example/zopeintegration/configure.zcml', '/Users/maurits/community/plone-coredev/6.2/src/plone.autoinclude/test-packages/example.different2/src/example/somethingelse2/meta.zcml'}

So this does not quite work, and my own PR #30 does.

But indeed we should move away from pkg_resources, so I appreciate your effort, and we should continue this at some point in the near future.