In response to feedback, and especially to the points about ecosystem
impacts and encouraging users to move package metadata out of the
current standard location, remove support for Dependency Group
Includes in the `[project]` table.

This is now listed with rationale in Rejected Ideas, and several
sections have been updated for correctness to fit this removal.

In "PEP 735: Dependency Groups", the defined semantics for a potential
name conflict between an extra and a group discouraged tools from
raising errors on such conflicts.

In the revised text, this is left completely open to implementers, and
it's more clearly likely and probable that some tools will error in
these cases. Users are still discouraged from creating this situation,
regardless of tool behaviors.

`uv` presented feedback that they allow for users to directly
reference content in the `[project]` table, even for non-package
projects. As a result, the text regarding extras as package metadata,
and the conflcit this presents with non-package projects, needs
clarification.

Simply add a section which calls this out explicitly and clarifies.

Some of the PEP feedback indicated that readers did not find the link
to the Use Cases section. Provide a second link in the Motivation
section, so that it is earlier in the document.

This text was based on a misunderstanding of information shared in the
discourse forum. There is no need to keep it.

This is extending the PEP text for an ancillary point of confusion --
there are tool users for `uv` (and possibly `pip-tools`) who do not
understand that `requirements.in` is a convention that does not
represent a significantly distinct file-format.

Discussing the difference between these only lengthens the already
very-long PEP text, and does not add value vis-a-vis community
standards. Given that neither `requirements.in` nor `requirements.txt`
have clear specifications, other than "what `pip` accepts", keeping
with the older and more standardized naming keeps things shorter and
simpler.

Per suggestion and with additional clarification.

Move the sections on `[project]` includes and `[build-system]`
includes to Deferred Ideas, as an added section.

Restore the removed use case for this, as a subsection of one of these
Deferred Ideas.

Also, per suggestion, remove an unnecessary phrase ("defined below" in
a context where it is _immediately_ followed by the relevant
section).