Better identification of broken Distribution objects

首先使用 issue 中的 Python 3.9 Docker 和 tox 命令复现失败，然后阅读 importlib_metadata/compat/py39.py 中的 normalized_name 和 importlib_metadata/init.py 中的 normalize。跟踪仅包含 _path 的 Distribution 如何进入名称规范化流程；当缺失或无效的名称能够提供有用的信息来识别失败的 Distribution，且相关测试通过时，即表示完成。

Currently, I'm getting an error on Python 3.8 and 3.9 in https://github.com/pypa/build/pull/820:

docker run --rm -it python:3.9 bash
pip install tox
git clone https://github.com/pypa/build
cd build
tox -e py39 -- -k test_metadata_path_no_prepare -v

...
File "/build/.tox/py39/lib/python3.9/site-packages/importlib_metadata/compat/py39.py", line 23, in normalized_name
    return Prepared.normalize(getattr(dist, "name", None) or dist.metadata['Name'])
  File "/build/.tox/py39/lib/python3.9/site-packages/importlib_metadata/__init__.py", line 889, in normalize
    return re.sub(r"[-_.]+", "-", name).lower().replace('-', '_')
  File "/usr/local/lib/python3.9/re.py", line 210, in sub
    return _compile(pattern, flags).sub(repl, string, count)
TypeError: expected string or bytes-like object

The problem is dist.__dict__={'_path': PosixPath('/build/tests/packages/test-no-prepare/test_no_prepare.egg-info')}. I think this is tripping up on tests/packages/test-no-prepare/test_no_prepare.egg-info/ and the local backend. But I don't know what updated to cause this to start happening.

Regardless of the solution, though, I think the handling here for a missing name should be better, there wasn't any useful info in the error message to tell me about the dist that was failing. I had to add print(f"{dist.__dict__=}") to see it. Or maybe the normalize name could return None, and let the failure happen elsewhere.