PEP 639: Further update per discussion, w/flat license key, etc.

[CAM-Gerlach]

At long last, the next (and hopefully final) round of substantive PEP 639 (PEP-639) updates has arrived, based on the consensus of recent (and by now, not so recent) community discussions.

See the preview here

I was initially going to make the Terminology section into a Sphinx glossary and link the terms on first and prominient uses for clarity, but to speed things up and avoid scope creep I've deferred that to an immediate followup.

In a final followup PR, I'll reduce the length of the PEP by a further ≈two thirds (on top of substantial earlier reductions and modest further trimming here) by moving all the appendices, the "Mapping License Classifiers to SPDX Identifiers" section (previously normative) and the full rejected ideas section (minus a concise summary of the most-discussed ones) to separate ancillary files linked from the appropriate places. Since I've already set the stage for that with the work in #2531, it should be a fairly straightforward and mostly mechanical change once this PR is editorially reviewed and merged.

Substantive content changes:

• Instead of adding a new top-level license-expression key for the license expression in the [project] table of the pyproject.toml source metadata, the PEP now specifies using the top-level string value of the license key for this purpose, which PEP 621 (PEP-621) reserved it for, and updates the license-expression and license key specs, and the examples, rejected ideas and other sections accordingly.
• Since this makes it not possible to convert a legacy license key to the new License-Expression field at build time, and that's not really advisable anyway, it drastically simplifies the normative Converting Legacy Metadata section to just a single normative statement, and updates/removes other mentions of it accordingly, and the (already rather unhelpful) example
• Likewise, it simplifies and refines the guidance in the Mapping classifiers to SPDX identifiers section to be more general and less focused on build time, and also allows tools to ignore redundant parent classifiers (Note: This section will be moved to an external appendix in the next PR, but I didn't move it here for ease of review)
• The license_files directory was renamed to licenses at the request of @brettcannon and to simplify things a touch
• The specified handling of the license.file key was a bit confused by a (apparently quite common) misunderstanding about how the specified file is used (to inject its text directly under the License field in core metadata, rather than included in distribution archives or its path specified in metadata) due to it being rather underspecified in PEP 621, which this revision corrects.
• Bump the core metadata version to reflect acceptance of PEP 685 as Metadata 2.3
• Bump the SPDX license list version to be up to date

Significant non-normative/editorial changes:

• Mention implementation of drafts of this PEP in Hatch and Setuptools, and also describes previous efforts in Setuptools and Wheel that this PEP builds on more clearly, as suggested on the discussion thread
• Reference the canonical project [source] metadata PyPA spec instead of PEP 621, use clearer language surrounding that, and update a few references to other PEPs
• Revise the Motivations and Rationale to be less duplicative/redundant, more balanced and focused more specifically on their respective areas (providing some background and describing the problem, and introducing and justifying the proposed solution, respectively)
• Add a short blurb making explicit the use of RFC 2119 terminology (MUST, SHOULD, etc)
• Update PEP headers
• Various other minor revisions to correct typos and other issues, clarify and simply the text, and improve source formatting.

[JelleZijlstra]

Generally looks good, but a few notes on wording

[JelleZijlstra]

All this is one sentence. It would be more readable if split up.

[CAM-Gerlach]

Done ✔️ , I made that change, caught a few other stray spaces and such and squashed it into the same commit with yours.

[CAM-Gerlach]

Going to go ahead and merge this soon unless anyone has any last feedback, and iterate further a final followup PR making the Terminology definitionlist into a glossary, linking the terms on first use in sections for clarity, adding additional ref targets, and taking advantage of the new Intersphinx support added in #2702 to link to the PyPA specs and glossary terms, making them simpler, more robust and more useful, as well as making it easier to both convert it to a PyPA spec, and review the same (avoiding some of the problems in pypa/packaging.python.org#1111).

[CAM-Gerlach]

Merging now, I'll make any last changes (glossary, intersphinx, etc) in a followup.