[debt] Remove distutils hack

[jaraco]

Although distutils_hack is deprecated (#4137), there are still consumers that rely on the presence of distutils or access to its internals to conduct some business (see the distutils deprecation tag).

One aspect of this deprecation is to make the compilers module independent. Some work has been done on that by moving the compilers into distutils.compilers with the intention of eventually releasing that functionality as a third-party compliers.C package. Before that can happen, those modules need to be decoupled from distutils (and any interaction with distutils made a formal interface). There is some work already drafted (and even some merged) in https://github.com/pypa/distutils/tree/debt/compilers-deps.

Let's finish the work. Let's provide solutions for each of the distutils deprecation issues and decouple the compilers functionality. Some backward incompatibility may be acceptable, but we'll aim to remain compatible where feasible.

Plan

End-state: compilers.C fully decoupled from distutils and extractable as a standalone distribution under jaraco/compilers.C. compilers stays a namespace package so future compilers.<lang> siblings can be added. Removal of the top-level distutils hack is gated on these replacements shipping.

The work is broken into the sub-issues tracked above. Grouped into phases:

Phase 1 — Decouple compilers.C from distutils internals (pypa/distutils)

• Fix mingw bare -O optimization flag under -m32 #5265 — Fix mingw bare -O under -m32 (closes mingw64 win32 compilation issue - does not accept "-O" #4873)
• Inline logging in compilers.C (drop distutils._log) #5266 — Inline logging (drop distutils._log)
• Vendor small build/filesystem helpers into compilers.C #5267 — Vendor small build/filesystem helpers
• Replace version & platform-compat shims in compilers.C #5268 — Replace version & platform-compat shims
• Move compilers.C to stdlib sysconfig + injection shim (investigation) #5269 — Move to stdlib sysconfig + injection shim (investigation)
• Consolidate compiler exceptions at compilers.errors #5270 — Consolidate compiler exceptions at compilers.errors

Phase 2 — Public API (pypa/distutils)

• Public API for compilers.C #5271 — Public API for compilers.C (resolves Expose compiler classes for customized use #2806, Equivalent for distutils.ccompiler.new_compiler needed #4540)

Phase 3 — Migration docs (pypa/setuptools)

• Fill migration-guide gaps for ccompiler / new_compiler / get_platform #5272 — Fill migration-guide gaps for ccompiler / new_compiler / get_platform (resolves doc half of Expose compiler classes for customized use #2806)

Phase 4 — Retire the hack (pypa/setuptools; gated on Phases 1–3)

• Retire the distutils hack (remove top-level distutils injection) #5273 — Retire the distutils hack (see also Deprecate SETUPTOOLS_USE_DISTUTILS=stdlib #3625, Deprecate usage of import distutils except internal to Setuptools #3622)

Extraction

• Stand up standalone jaraco/compilers.C distribution #5274 — Stand up standalone jaraco/compilers.C distribution

Design decisions: the standalone library defaults to stdlib sysconfig (behavior change acceptable), with a possibly-deprecated shim for distutils/setuptools to inject specialized behavior; no setuptools re-export layer — consumers import from the compilers package directly.