Add # sage_setup: distribution directives to all files, remove remaining # coding: utf-8

[saraedum]

This PR recreates https://github.com/sagemath/sage/pull/36964 by @mkoeppe after it was reverted, see https://github.com/sagemath/sage/pull/37796.

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These directives at the top of the file inform developers about the intended assignment of modules to pip-installable distributions.

As of this PR, there should be no change to the existing distributions (sagemath-categories...) nor the monolithic build of the Sage library.

• Cherry-picked from #35095

:memo: Checklist

• [x] The title is concise, informative, and self-explanatory.
• [ ] The description explains in detail what this PR is about.
• [ ] I have linked a relevant issue or discussion.
• [ ] I have created tests covering the changes.
• [ ] I have updated the documentation accordingly.

:hourglass: Dependencies

• Depends on #36951 (merged here)
• Depends on ~#36676~ #37900 (merged here)
• Depends on #37667 (merged here)
• Depends on #38031 (split out from here)