The crux here is supporting a version of Pip that works in 3.12. There
is no such released version yet; so this change adds an unreleased
Pip version but goes to some length to hide this version from users
and make it only activatable by those in the know / CI. What follows
is fixing or adjusting many tests. The result is Pex known to work with
Python 3.12 ahead of its release by several months and the spectre of
Pex 3 / a Pex branch split, etc., dispelled.

It turns out Pex can still ship supporting Python 2.7, 3.5, etc.
along side supprting 3.12. The main trick here is to use
`python3.12 -mvenv` to spirit up a bootstrap Pip that works at least
enough to install the unreleased Pip that truly works with Python 3.12.
Previously all Pip version bootstrapping was handled exclusively by the
vendored Pip.

Move from `installed_wheel_zips` to `packed_wheels` for the packed
layout wheel zip cache. This avoids reading old bad `../` zip entries
under Python 3.12+ while allowing co-existence.