You can now specify `--scie {eager,lazy}` when building a PEX file and
one or more additional native executable PEX scies will be produced
along side the PEX file. These PEX scies will contain a portable CPython
interpreter from [Python Standalone Builds][PBS] in the `--scie eager`
case and will instead fetch a portable CPython interpreter just in time
on first boot on a given machine if needed in the `--scie lazy` case.

Although Pex will pick the target platforms and target portable CPython
interpreter version automatically, if more control is desired over which
platforms are targeted and which Python version is used, then
`--scie-platform`, `--scie-pbs-release`, and `--scie-python-version` can
be specified.

Closes pex-tool#636
Closes pex-tool#1007
Closes pex-tool#2096

[PBS]: https://github.com/indygreg/python-build-standalone

This stresses the full matrix of basic cases (no `--scie-*` options).

Exercise platform / interpreter auto detection as well as explicit
restriction via `--scie-platform`. Also open up control via `PEX_*`
env vars: there was no need to mask thse for proper scie operation
and the end result is a scie that works exactly like the PEX it was
built from.