replace pkg_resources with importlib.resources/metadata for setuptools 82 compat

[tagaws]

setuptools 82.0.0 removed pkg_resources (see
https://setuptools.pypa.io/en/latest/history.html#v82-0-0).
This caused ModuleNotFoundError: No module named 'pkg_resources' on any cfn command.

Changes

• data_loaders.py: replace pkg_resources.resource_stream with importlib.resources.files (stdlib 3.9+, with importlib_resources backport for <3.9)
• plugin_registry.py: replace pkg_resources.iter_entry_points with importlib.metadata.entry_points
• setup.py: add importlib_resources>=5.0 as conditional dep for Python <3.9
• Updated unit tests and added tests/functional_importlib_compat.py integration tests

Testing

• 99/99 unit + integration tests passing on Python 3.10 with setuptools 82.0.1 installed
• cfn submit --dry-run confirmed working end-to-end
• Companion fix for the build packaging issue: [cloudformation-cli-python-plugin PR link here]

Related

• Another PR needed in cloudformation-cli-python-plugin to pin setuptools<82 in _make_pip_command(A fix in codegen.py is needed to pin <82 as the build overides this)

[tagaws]

#1117