sagemathgh-37500: `sage --package`, `sage-get-system-packages`: Support PURLs `pkg:pypi/DISTRO-NAME`, obtain dependencies of wheels from PyPI
    
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We make it possible to refer to Python packages via their PURL (see
[draft PEP 725](https://peps.python.org/pep-0725/#concrete-package-
specification-through-purl)) instead of their SPKG name.

For now a string of the form `pkg:pypi/DISTRO-NAME` is simply a nickname
for the (unique) SPKG that has DISTRO-NAME in their `install-
requires.txt` or `requirements.txt`. The scheme can also be omitted:
`pypi/DISTRO-NAME` also works. And we also map `pkg:generic/PACKAGE-
NAME` to `PACKAGE_NAME`.

Based on code by @culler, `sage --package create --pypi` now also fills
`dependencies` from the PyPI metadata of wheel packages. When some of
the Python dependencies obtained in this way do not have SPKGs yet, they
are also automatically created.

- Preparation for sagemath#31136.
- Split out from sagemath#37250.


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### ⌛ Dependencies

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URL: sagemath#37500
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Marc Culler, Matthias Köppe