PEP 771: Amendments following discussion

[astrofrog]

Following the initial publication of PEP 771, there has been a lot of discussion in this DPO thread. This PR makes changes to PEP 771 that hopefully addresses most of the concerns there.

The main changes are:

• package[] is now considered to be the primary way to specify that a package without any default extras is being requested
• The alternative approach of splitting packages into two has now been added, and given a reasonably extensive section as it appears to be the main alternative
• Added text regarding packages that have both multiple backends and frontends and might want at least one default for each
• Added a reference to PEP 751 since that PEP in principle solves the current pip freeze issues that might arise from PEP 771
• Added text on how maintainers should document default extras

In addition to this, there are other smaller additions that aim to address some of the concerns in the DPO discussion.

Once this PR is merged, I'll then open a new discussion thread and open a trivial PR here to add the link to the PR.

cc @pradyunsg @DEKHTIARJonathan @henryiii

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