pypi: Enhance guidance on how to reconstruct a "file_name" qualifier from inspecting installed Python packages

[pombredanne]

As a follow up to:

• https://github.com/package-url/purl-spec/pull/170#issuecomment-2477108437 we should provide guidance to tools authors on how to reconstruct a file_name when inspecting an installed Python package.

So to recap, I think we can use the existing file_name qualifier as-is, but we would need to provide guidance on how a tool can reconstruct this from an installed distribution. Can you update this PR along these lines, using file_name and providing some guidance on its usage, including pointers on how to reconstruct a filename from an installed PyPI wheel? This would be wonderful! (Sorry for it taking so long!)

[matt-phylum]

The formats are:

• source distributions: https://packaging.python.org/en/latest/specifications/source-distribution-format/#source-distribution-file-name
• binary distributions: https://packaging.python.org/en/latest/specifications/binary-distribution-format/#file-name-convention

I don't know if pip keeps a record of where the package came from so it may be difficult to figure out the appropriate name from an install package. Packages that were downloaded as a source distribution may appear to have been installed from a binary distribution.