Cache pip.pex.

[jsirois]

Previously, the pip.pex was re-created once per Pex CLI run. Now it is
created once per unique Pex distribution.

Perf result difference for a single distribution PEX indicates this
shaves off ~600ms of overhead for a warm cache (after the 1st run of the
Pex CLI at a given version):

Before (Pex 2.1.7):

$ multitime -s 2 -n 10 python -m pex psutil -o psutil.pex
===> multitime results
1: /home/jsirois/.venv/pex1.6/bin/pex psutil -o psutil.pex
            Mean        Std.Dev.    Min         Median      Max
real        2.044       0.008       2.030       2.046       2.060
user        1.690       0.038       1.636       1.700       1.751
sys         0.220       0.039       0.164       0.210       0.273

After:

$ multitime -s 2 -n 10 python -m pex psutil -o psutil.pex
===> multitime results
1: python -m pex psutil -o psutil.pex
            Mean        Std.Dev.    Min         Median      Max
real        1.365       0.070       1.328       1.339       1.571
user        1.070       0.019       1.030       1.074       1.105
sys         0.139       0.015       0.116       0.137       0.170

[cosmicexplorer]

This is awesome!!! I really like the isolated() API.