Decouple the C compilers from distutils build/filesystem helpers

[jaraco]

Removes the C compilers' dependence on distutils' build/filesystem helpers, per the analysis in pypa/setuptools#5267 (Phase 1 of pypa/setuptools#5264).

What changed

Vendored into the compilers namespace package (stdlib-only, siblings of the logging.py from #5266):

• compilers/_modified.py — newer/newer_group mtime helpers.
• compilers/_util.py — split_quoted, vendored verbatim (a switch to shlex.split is not behavior-equivalent and is left for later).

Subprocess execution reworked into a modern call + deprecated spawn:

• New Compiler.call(cmd, *, env=None, **kwargs) — the modern entry point: logs the command, injects the macOS deployment target, and runs subprocess.check_call, letting native subprocess exceptions propagate.
• Compiler.spawn is now a deprecation shim — it warns, delegates to call, and translates subprocess/OSError failures to DistutilsExecError (imported late, to keep the package off distutils.errors at module scope).
• Internal call sites across unix/cygwin/zos/msvc moved to call() and now catch subprocess.CalledProcessError/OSError. MSVC's PATH injection moved from a spawn override to a call override.
• The macOS env injection moved out of distutils.spawn into compilers/platform/macos.py (_inject_ver) — the compiler path is the only place it applies (verified: no non-compiler spawn call site invokes a compiler/linker).
• The previously-vendored compilers/spawn.py is removed.

distutils.spawn.spawn reduced to a thin wrapper — it now only logs, forwards cmd/**kwargs to subprocess.check_call, and maps OSError/CalledProcessError to DistutilsExecError; no shutil.which (subprocess searches PATH itself) and no macOS injection. Command.spawn no longer forwards the now-defunct search_path.

Modernized in place — the public Compiler methods keep their signatures for backward compatibility, but their bodies now use the stdlib:

• mkpath → os.makedirs(name, mode, exist_ok=True) (empty-name guarded).
• move_file → shutil.move.
• execute → trivial log-and-call, no longer used internally.

Eliminated — the cygwin .def-file write is inlined (pathlib.Path.write_text), removing distutils.file_util.write_file and the execute indirection. Also removed the MSVC _fallback_spawn monkeypatch shim (numpy.distutils <1.19, per #15) and its now-dead pytest filter.

Net: compilers/C/* no longer import distutils.spawn, dir_util, file_util, _modified, or util.execute/split_quoted.

Deliberately out of scope

Two transitive couplings remain, by design, as they belong to sibling sub-issues:

• the vendored _modified / deprecated spawn still reference Distutils*Error (native compilers.errors is #5270 / S6);
• _inject_ver still calls distutils.util for the target-version helpers (platform/util decoupling is #5268 / S4).

Verification

• Full distutils suite: 246 passed, 22 skipped. Compiler + build_ext + spawn subset green.
• Verified call() raises native exceptions without warning; spawn() warns and translates to DistutilsExecError; distutils.spawn.spawn still translates.
• ruff check --select F,I clean on all changed files.

🤖 Generated with Claude Code

[jaraco]

Use from ..platform import macos, so that the macos name is reflected in the call below.

[jaraco]

Let's unify this behavior with distutils.spawn.spawn, either by extracting a context manager to do the exception translation or by having "inner" spawn that's called after the logging is done.