Come up with a plan for installing wheels in pexes post wheel 0.31.1

[jsirois]

After 0.31.1 the wheel.WheelFile.install API we use here is gone:
https://github.com/pantsbuild/pex/blob/ac7aab86e90cfdff43e0b05c6a3500a6cb354547/pex/pex_builder.py#L279-L291

See pypa/wheel#262

[jsirois]

The WheelFile.install code is not super involved even though it looks extensive at 1st glance:
https://github.com/pypa/wheel/blob/3dc261abc98a5e43bc7fcf5783d080aaf8f9f0cf/wheel/install.py#L275-L397

The answer may be to just buckle down and implement install ourselves.

[kwlzn]

The answer may be to just buckle down and implement install ourselves.

makes sense to me for now until pypa/wheel#262 manifests - or perhaps we could both fork and attempt to upstream that impl as contrib solution for pypa/wheel#262.

[jsirois]

This was solved in Pex 2.0.0 by vendoring Pip and using it to perform installs in #788