Allow Pip rules to use wheels

[jayvdb]

Wheels dont work for compiled extensions, however they should be fine for pure python packages. An option to enable wheels would help with packages without sdists. Also should be quicker.

[indygreg]

Yes, we should be smarter about allowing source-only wheels.

Also, I do want to support binary wheels at some time. Once PyOxidizer supports more granular control over binary generation, it should be possible to produce a libpython equivalent and enable compiled extension modules in binary wheels to just work. This won't get you standalone executables. But it makes a lot of packaging problems go away and PyOxidizer significantly more usable as a result.

[jayvdb]

As a heads-up, I have a substantial patch for this, nearing completion. I see you have been busy re-arranging everything, so I'll need to catch up. Hopefully we're not overlapping much.

[jayvdb]

#199 is the foundation of using wheels.
Installing wheels (pure python) with --target introduces new error situations, which would make user support more difficult.