Document that Requires-Dist can use explicit URLs

[godlygeek]

The core-metadata documentation currently implies that Requires-Dist can only use name-based specifiers, but then references PEP 508 which allows for URL based specifiers. Update the documentation to reflect this alternative specifier format.

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📚 Documentation preview 📚:

• https://python-packaging-user-guide--1783.org.readthedocs.build/en/1783/specifications/core-metadata/#requires-dist-multiple-use
• https://python-packaging-user-guide--1783.org.readthedocs.build/en/1783/specifications/version-specifiers/#direct-references

[ncoghlan]

This section should reference https://packaging.python.org/en/latest/specifications/dependency-specifiers/ rather PEP 508 directly.

Any mention of direct URLs should point to their definition in https://packaging.python.org/en/latest/specifications/version-specifiers/#direct-references and note that they are not permitted on PyPI (but are fine for use in packages that are only uploaded to org specific index servers).

The description of valid names should link to https://packaging.python.org/en/latest/specifications/name-normalization/#name-format

[godlygeek]

I believe I've addressed all of your feedback @ncoghlan, please take another look