Remove empty default build tree left behind by bdist_wheel

[CAOShurong]

Summary

Fixes #5134 (the build/bdist.* leftover part — the separate egg-info question discussed in that issue is out of scope here).

With the default options, a wheel build leaves an empty directory chain behind:

$ python -m build --wheel
$ tree -d
.
├── build
│   └── bdist.win-amd64   <-- empty
└── dist

The cause: bdist_wheel removes only its own scratch directory (bdist_dir = build/bdist.<plat>/wheel), so the parent directories it created along the way survive as empty shells.

Change

After removing bdist_dir, prune directories that are still empty walking up the default chain ([build] build_base → bdist.<plat>). The pruning stops at the first non-empty directory and is guarded on both sides:

• a user-supplied --bdist-dir (anything other than the default <bdist_base>/wheel) is never touched;
• a customized [build] build_base disables pruning entirely.

A non-empty build/ containing real artifacts (e.g. from build_ext) is preserved as before.

Testing

• New tests in setuptools/tests/test_bdist_wheel.py:
  • test_bdist_base_removed_when_empty: default build leaves no build/ behind (verified RED without the fix: 1 failed);
  • test_bdist_base_preserved_when_not_empty (parametrized): custom --bdist-dir, plus stale file / subdirectory left inside the base — never removed.
• Full suite: 32 passed, 2 skipped on current main.
• End-to-end reproduction of the issue scenario with python -m build --wheel: before the fix build/ + build/bdist.win-amd64 remain; after the fix only dist/ remains.

[mergify]

Tick the box to add this pull request to the merge queue (same as @mergifyio queue).

• Queue this pull request