Clarify definition of native and implicit namespace packages

[stevepiercy]

This pull request clarifies the definition of native and implicit namespace packages as synonyms, and adds a glossary term for them.

From a discussion in Discord at https://discord.com/channels/935215565872693329/935215566334079058/1451178701956976853, by an initially very confused reader who's relatively new to the terminology.

Feedback welcome. Thank you for your consideration.

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📚 Documentation preview 📚: https://python-packaging-user-guide--1979.org.readthedocs.build/en/1979/

[ncoghlan]

Something that occurred to me while reviewing the complementary PR at #1882 is that it would make sense to mutually cross link the new glossary entry with the more detailed description at
https://packaging.python.org/en/latest/guides/packaging-namespace-packages/#native-namespace-packages

[stevepiercy]

Something that occurred to me while reviewing the complementary PR at #1882 is that it would make sense to mutually cross link the new glossary entry with the more detailed description at https://packaging.python.org/en/latest/guides/packaging-namespace-packages/#native-namespace-packages

Done in eae76d7

[stevepiercy]

Spoke too soon. I assumed there was an existing target, so I added one in bd73c03. Sorry for the noise.

There's still our little friend at:

packaging.python.org/source/guides/packaging-namespace-packages.rst

Line 162 in 11477a6

with packages already using this method. Also, :doc:`pkg_resources <setuptools:pkg_resources>`

causing this warning:

https://github.com/pypa/packaging.python.org/actions/runs/22292222102/job/64481563875?pr=1979#step:5:66

It appears that that page no longer exists in the setuptools docs. I made a guess with a0c8b87. That appears to have resolved both issues.