Remove use of pkg_resources

[mattwthompson]

pkg_resources is officially-officially deprecated

$ grep -r pkg_re openff                               10:22:46  ☁  main ☂
openff/evaluator/plugins.py:import pkg_resources
openff/evaluator/plugins.py:    for entry_point in pkg_resources.iter_entry_points("openff_evaluator.plugins"):
openff/evaluator/datasets/taproom/taproom.py:import pkg_resources
openff/evaluator/datasets/taproom/taproom.py:        for entry_point in pkg_resources.iter_entry_points(group="taproom.benchmarks"):
openff/evaluator/tests/test_plugins.py:import pkg_resources
openff/evaluator/tests/test_plugins.py:    distribution = pkg_resources.Distribution(__file__)
openff/evaluator/tests/test_plugins.py:    valid_entry_point = pkg_resources.EntryPoint.parse(
openff/evaluator/tests/test_plugins.py:    bad_entry_point = pkg_resources.EntryPoint.parse(
openff/evaluator/tests/test_plugins.py:    pkg_resources.working_set.add(distribution, "dummy_1")
openff/evaluator/tests/test_plugins.py:    pkg_resources.working_set.add(distribution, "dummy_2")
openff/evaluator/utils/utils.py:    from pkg_resources import resource_filename

IIRC the replacement for its entrypoints is found in importlib-metadata