Add PEX info to the PEX repl.

[jsirois]

It should now be clear when you've dropped into a PEX repl (vs. a plain
Python repl). This may help stem some of the confusion brough about by
running pex with no arguments. In addition, you gain some
introspection capabilities via the pex_info REPL command.

Fixes #157

[jsirois]

For the #157 OP case of running a PEX pex with no args:

And the more general case of a normal PEX repl built via:

:; pex --interpreter-constraint ">=3.8" --pip-version latest requests --sh-boot -orequests.pex

[jsirois]

Reviewers, another big one - but broken up into its bits anyhow. Thanks in advance.

This will probably be the last of these. It's not clear to me review makes much sense anymore except when I actually have questions you might help answer. I don't really like lobbing these things at you guys and I'm sure you don't enjoy being lobbed at.

[benjyw]

Splitting this into multiple coherent commits made it quite straightforward to review, thanks for that!

[benjyw]

Reviewers, another big one - but broken up into its bits anyhow. Thanks in advance.

This will probably be the last of these. It's not clear to me review makes much sense anymore except when I actually have questions you might help answer. I don't really like lobbing these things at you guys and I'm sure you don't enjoy being lobbed at.

I am happy to do any code reviews that are actually useful to you! Or when you think it might improve something. But just rubber-stamping makes little sense, I agree.