Adopt the standalone compilers packages

[jaraco]

Adopt the standalone compilers distributions that were extracted from distutils, completing the extraction side of the compilers-decoupling epic (pypa/setuptools#5264, pypa/setuptools#5274).

What

• Declare runtime dependencies on compilers.C, compilers.common, and compilers.errors (published from coherent-oss).
• Delete the vendored distutils/compilers/ tree (its contents now live in, and are CI-tested by, those packages).
• Rewire the ~10 internal references from distutils.compilers.* to the top-level packages:
  • the compiler shims (ccompiler, unixccompiler, _msvccompiler, cygwinccompiler, zosccompiler) → compilers.C.*
  • errors → compilers.C.errors + compilers.errors
  • util → compilers.common.platform
  • compat.numpy → compilers.C.base
• The compiler tests move out with the tree; conftest.py drops the now-orphaned suppress_path_mangle fixture and repoints disable_macos_customization at compilers.common.platform.macos.

The public shims are unchanged in behavior — distutils.ccompiler.CCompiler is compilers.C.base.Compiler, distutils.errors.CCompilerError is compilers.errors.Error, distutils.util.get_platform() routes through compilers.common.platform.detect, etc.

There's no dependency cycle: compilers.C imports distutils only lazily on deprecated paths (deprecated Compiler.spawn, debug_print) and deliberately doesn't declare it (coherent-oss/compilers.C#1), so this is a clean one-way distutils → compilers.C dependency.

Verification

With the three packages installed, the full suite passes (220 passed, 9 skipped) and ruff is clean.

Notes

• No newsfragment here by design — the changelog entry will be added when this syncs into setuptools.
• Companion setuptools work is required: setuptools vendors distutils as setuptools/_distutils, so once distutils depends on compilers.C, setuptools must bundle or depend on compilers.C/compilers.common/compilers.errors for its _distutils to keep working. That's the pivot into Phase 4 (Retire the distutils hack (remove top-level distutils injection) setuptools#5273) and will be handled separately.

[jaraco]

CI is green except the expected ci_setuptools integration failure:

setuptools/_distutils/errors.py: from compilers.C.errors import ...
ModuleNotFoundError: No module named 'compilers'

That job drops the current distutils into setuptools' vendored _distutils, whose (synced) errors.py now imports from compilers.C — but setuptools' environment doesn't have the compilers.* packages installed. Resolving it is the companion setuptools change (bundle or depend on compilers.C/compilers.common/compilers.errors), i.e. Phase 4 of pypa/setuptools#5264 (pypa/setuptools#5273). Everything else — full test matrix, mypy, docs, coverage — passes.