Retire the distutils hack (remove top-level `distutils` injection)

[jaraco]

Part of the compilers decoupling epic #5264 (Phase 4). setuptools repo. Gated on Phases 1–3 shipping.

With replacements available (compilers.C public API + migration docs), retire the hack:

• Escalate the SETUPTOOLS_USE_DISTUTILS=stdlib and import-order deprecations from Deprecate distutils_hack and support for 'import distutils' before 'import setuptools' #4137 to removal (see also Deprecate SETUPTOOLS_USE_DISTUTILS=stdlib #3625 and Deprecate usage of import distutils except internal to Setuptools #3622)
• Remove _distutils_hack.DistutilsMetaFinder and stop injecting the top-level distutils name
• Triage-close downstream deprecation noise (e.g. python12 ubutu 20.04 build horovod 0.28.1  #4655 and generic "distutils deprecation warning" reports), pointing at the migration guide