bootstrap_pex_env should support PEX_PATH

[kwlzn]

currently, using pex_bootstrapper.bootstrap_pex_env() to bootstrap imports from an external pex inside of a running python intepreter does not appear to natively handle PEX_PATH:

[omerta xxx]$ pex requests -o requests.pex
[omerta xxx]$ pex flask -o flask.pex
[omerta xxx]$ python2.7
Python 2.7.13 (default, Mar  2 2017, 16:22:01) 
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.42.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import os, sys
>>> pex_filename = 'flask.pex'
>>> sys.path.insert(0, os.path.join(os.path.abspath(pex_filename), '.bootstrap'))
>>> from _pex import pex_bootstrapper
>>> os.environ['PEX_PATH'] = os.path.abspath('./requests.pex')
>>> pex_bootstrapper.bootstrap_pex_env(pex_filename)
>>> import requests
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named requests

a viable workaround for the moment is to call bootstrap_pex_env once per pex:

>>> pex_bootstrapper.bootstrap_pex_env('requests.pex')
>>> import requests
>>>

but it'd be nice if this "just worked" in the same fashion of the typical pex runtime entrypoint.

[jsirois]

This is related to #525 where I ran into problems due to the non-uniformity of pex build and pex run re interpreter selection. I should cleanup #525 to include this.

[jsirois]

I worked hard to rid the need for Pex API use since Pex only supports its CLI interface. With the introduction of the __pex__ magic import hook in #1845, you can now do this instead:

:; pex requests -o requests.pex
:; pex flask -o flask.pex
:; python
Python 3.11.9 (main, Apr 26 2024, 19:20:24) [GCC 13.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import os, sys
>>> sys.path.insert(0, os.path.abspath("flask.pex"))
>>> os.environ['PEX_PATH'] = os.path.abspath("requests.pex")
>>> import __pex__
>>> import requests
>>> import flask
>>> requests.__file__
'/home/jsirois/.pex/installed_wheels/70761cfe03c773ceb22aa2f671b4757976145175cdfca038c02654d061d6dcc6/requests-2.32.3-py3-none-any.whl/requests/__init__.py'
>>> flask.__file__
'/home/jsirois/.pex/installed_wheels/34e815dfaa43340d1d15a5c3a02b8476004037eb4840b34910c6e21679d288f3/flask-3.0.3-py3-none-any.whl/flask/__init__.py'
>>>