Graduate wheel-building code out of //experimental

[pstradomski]

PR Checklist

Please check if your PR fulfills the following requirements:

• Does not include precompiled binaries, eg. .par files. See CONTRIBUTING.md for info
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe: Move wheel-building rules out of //experimental.

Does this PR introduce a breaking change?

• Yes
• No

No, the rules are still re-exported at the old location

FIXES #374

[thundergolfer]

LGTM.

Now that this is out of experimental/, py_wheel probably warrants a mention in the README. But that is not a big deal and can be sorted out in a followup.

[UebelAndre]

Could a new release be made for this change? Or what goes into making a new release?