Create a new wheel_builder module and move WheelBuilder+friends

[pradyunsg]

Moves WheelBuilder and friends into a dedicated module.

These are already basically independent from the rest of pip._internal.wheel so doing so makes a lot of sense IMO. Reduces ~500 lines from wheel.py.

Refactor done w/ PyCharm's refactoring tools + cleanup.

[chrahunt]

This is a good thing to move out IMO. I would also move the tests.

[pradyunsg]

This is already a pretty huge PR as is, so I'm weary of adding more things to it. I'd prefer to move tests in a follow up.

[pradyunsg]

Actually, you're right -- there's only 1 test for WheelBuilder (ahahahahahaha) and that's failing already. I'll move that into it's own file, as a reminder that we need to expand these tests.

[sbidoul]

@pradyunsg it would be cool if #7285 could go be merged before, as it's my second attempt and the previous one was also killed by a refactoring underneath it. 🙏

[pradyunsg]

@sbidoul Yep yep -- that's on my radar. Don't worry, I don't plan on merging this change before that PR. :)

Edit: To be clear, even if I do, I don't expect there to be any major issues or merge conflicts with this PR (a git merge master or git rebase master should be all that's needed to get upto speed).

[pradyunsg]

Updated after #7285 merged. :)