Optional metadata sans distutils

[wimglenn]

Same as #3289 but without the changes under _distutils (companion to pypa/distutils#138).

[abravalheri]

I would still wait for the changes to land in distutils (and then merged back in setuptools) before merging this PR, otherwise the end result will be warnings that don't make sense (or are even conceptually wrong) for the the user and a bad UX.

If all the merges happen, we are going with situation 2 described in #3289 (comment)

[jaraco]

I would still wait for the changes to land in distutils (and then merged back in setuptools) before merging this PR, otherwise the end result will be warnings that don't make sense (or are even conceptually wrong) for the the user and a bad UX.

I'll plan to merge these all in the same day and avoid that experience.

If all the merges happen, we are going with situation 2 described in #3289 (comment)

[if the user has set SETUPTOOLS_USE_DISTUTILS=stdlib], they will see some warnings that will not make sense (and even be conceptually unjustified), but the build will ultimately pass. That is a bad UX. Maybe we can see the bad UX in this case as a compromise?

I'd like to deprecate SETUPTOOLS_USE_DISTUTILS=stdlib soon, so that seems like a fair compromise. We may have to back out some of these warnings if they cause unmanageable disruption.