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.