Remove use of pkg_resources

pkg_resources is now official deprecated (https://github.com/pypa/setuptools/issues/3085). Its use in pex is causing this warning: ``` UserWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html. The pkg_resources package is slated for removal as early as 2025-11-30. Refrain from using this package or pin to Setuptools<81. import pkg_resources # vendor:skip ``` Can the functionality pkg_resources provides be replaced by importlib.resources?