Best match should be a pre-release if that's all the index has

[wimglenn]

Hi, I'm not 100% sure if this may be considered a bug, but if Unearth serves as a stable replacement for pip's PackageFinder API should be understood as meaning a drop-in replacement, then perhaps it is?

Pip will select a pre-release if that is all that is available in the index:

$ python3 -m pip --version
pip 24.2 from .venv/lib/python3.13/site-packages/pip (python 3.13)
$ python3 -m pip install --index-url=https://test.pypi.org/simple alphaonly123 
Looking in indexes: https://test.pypi.org/simple
Collecting alphaonly123
  Downloading https://test-files.pythonhosted.org/packages/b9/79/350172f075790dc4ae3127ca6b80c93d874db7341bc9f2a829cb24fa1044/alphaonly123-1.0a1-py2.py3-none-any.whl.metadata (284 bytes)
Downloading https://test-files.pythonhosted.org/packages/b9/79/350172f075790dc4ae3127ca6b80c93d874db7341bc9f2a829cb24fa1044/alphaonly123-1.0a1-py2.py3-none-any.whl (888 bytes)
Installing collected packages: alphaonly123
Successfully installed alphaonly123-1.0a1

But unearth won't, it needs to be asked explicitly for pre-releases to be included.

$ python3 -m pip install -q unearth                                           
$ python3
Python 3.13.0 (v3.13.0:60403a5409f, Oct  7 2024, 00:37:40) [Clang 15.0.0 (clang-1500.3.9.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from unearth import PackageFinder
>>> finder = PackageFinder(index_urls=["https://test.pypi.org/simple"])
>>> result = finder.find_best_match("alphaonly123")
>>> result.best
>>> result
BestMatch(best=None, applicable=<unearth.utils.LazySequence object at 0x101f2d6d0>, candidates=<unearth.utils.LazySequence object at 0x101e0acf0>)

Perhaps, with the default allow_prereleases=None mode, it should match pip - i.e. allow a pre-release if that is all the indices have available?