Vendor small build/filesystem helpers into `compilers.C`

[jaraco]

Part of the compilers decoupling epic #5264 (Phase 1).

Bring the leaf-level helpers the compilers use into the package so compilers.C no longer imports distutils for them:

• _modified.newer / newer_group
• dir_util.mkpath
• file_util.move_file / write_file
• spawn.spawn
• util.execute / split_quoted

Grouped PR, separate commits per helper family.

[jaraco]

Before proceeding with an implementation, let's analyze the usage of these helpers to determine if any of them can be eliminated (because they're unnecessary) or moved into the compilers package (because they're unused elsewhere). Remember that we want the compilers package to stand on its own with as little distutils-legacy cruft as possible, but we also want to provide compatabilty where reasonable for setuptools/distutils. One option to consider (for each behavior) is having compilers with a cleaner, more modern interface, but with shims for setuptools/distutils to inject legacy behaviors for compatibility. Provide options and make a recommendation for each.

[jaraco]

Analysis of the build/fs helpers

Before implementing, an analysis of the seven helpers (newer/newer_group, mkpath, move_file/write_file, spawn, execute, split_quoted) per the guidance.

Finding that reframes the "move it if unused elsewhere" option: none of the seven are unused elsewhere — every one still has live callers in distutils proper (command/*, cmd.py, util.py, archive_util.py, Extension). So "move out of distutils" isn't available for any of them; the realistic choices per helper are vendor a private copy into compilers, modernize onto stdlib, or eliminate/inline. Distutils keeps its own copies for its own commands regardless.

Second axis: four are public Compiler methods (spawn, mkpath, move_file, execute) — historical CCompiler API that downstream (numpy, Cython, setuptools) calls on compiler instances. The methods must stay for compatibility, but their bodies can be reimplemented. The other three (newer/newer_group, split_quoted, write_file) are internal-only.

Helper	Distutils cruft	Recommendation
mkpath	global path-cache + singledispatch + DistutilsFileError	Modernize method body → os.makedirs(name, mode, exist_ok=True); keep method
move_file	dry-run/verbose, cross-device logic	Modernize → shutil.move; keep method as thin compat
execute	near no-op since dry-run removal (log + call)	Keep trivial compat method, but stop using it internally
write_file	dry-run/verbose	Inline Path(def_file).write_text(...) (cygwin only); eliminates the helper and the execute indirection
split_quoted	none — pure stdlib, subtle semantics	Vendor verbatim into compilers; shlex.split is not behavior-equivalent (Windows paths/quotes), so not worth the risk now
newer/newer_group	DistutilsFileError, jaraco.functools	Vendor a small stdlib-only mtime helper into compilers
spawn	DistutilsExecError; also macOS MACOSX_DEPLOYMENT_TARGET env injection + an established monkeypatch/env= injection seam	Vendor a modern compilers spawn (subprocess + macOS injection + native error), exposing the injection seam — the one that fits the "clean interface + shim for legacy injection" model

Shape: vendor spawn, newer/newer_group, split_quoted as stdlib-only primitives in the compilers namespace package (siblings of the logging.py added in #5266); modernize mkpath/move_file/execute method bodies in place; inline+eliminate write_file.

Decisions taken for implementation:

• split_quoted vendored verbatim (exact semantics preserved); a later switch to shlex.split can be evaluated separately.
• S3 stays scoped: the vendored spawn/newer keep raising the existing Distutils*Error and spawn keeps the macOS injection via distutils.util for now. Migrating those to native compilers.errors is Consolidate compiler exceptions at compilers.errors #5270 (S6), and the macOS/platform util decoupling is Replace version & platform-compat shims in compilers.C #5268 (S4) — this PR removes only the spawn/dir_util/file_util/_modified/util.{execute,split_quoted} coupling.

PR to follow, as a grouped change with one commit per helper family.

[jaraco]

Completed by pypa/distutils#417 (merged).