Introduce PAR support / add an option to ship a PEX with Python included.

[jsirois]

With the advent of scies, Pex's can now be shipped as standalone native executables with the Python interpreter included without much effort. If Pex were to incorporate this feature, it might be spelled like:

--par {eager,lazy}

This would cause Pex to generate one or more binaries using scie-jump that either directly include the appropriate PBS Python archive or else lazily fetch it using ptex. The fiddly bit here will be selecting an appropriately matched PBS per Pex target Python / abbreviated --platform / --complete-platform.

[jsirois]

This has been a dream since ~day 1 of Pex, it was just too much work to consider. More recent surfacing of this desire has been in #636 and #1007.

[jsirois]

Noting that the last PBS release to support 3.7 is https://github.com/indygreg/python-build-standalone/releases/tag/20200822 and that support 3.7.9. Modern PBS releases support 3.8 through 3.11. So Pex could support --par for 3.8 through 3.11 trivially, but possibly also for 3.7 as well.

[jsirois]

Alright, since --par is a Googlism and the real backend is scie; I'm just rolling with --scie {lazy,eager} and a few --scie-* options for exact control of the PBS Python selected when needed. The initial support is just as a Pex CLI build option. If someone wants a pex3 tool to turn an existing PEX file into a scie, that is easy enough to add later. This change contains all the infrastructure to do that.