Support .egg-info dist metadata.

[jsirois]

This will allow inventorying system interpreters and venvs; which will
enable strict-checking support for PEX exlucions via --provided as
well as more general resolving from venvs (i.e.: adding a new option to
the --index/--find-links, --pex-repository and --lock set of
resolve repository options).

The new -d/--distributions option is added to
pex3 interpreter inspect to support creating the aforementioned
inventories.

[jsirois]

Work towards #2097 and #1361.

[jsirois]

The necessary motivational background is twofold.

1. System Python inventories:

Today, for example, on Ubuntu 22.04, just 1 system package (jeepney) is installed from a wheel; the rest from ~python setup.py install:

$ ls /usr/lib/python3/dist-packages/ | grep -E "\.(dist-info|egg-info)" | sort
PyGObject-3.42.1.egg-info
PyJWT-2.3.0.egg-info
PyYAML-5.4.1.egg-info
SecretStorage-3.3.1.egg-info
blinker-1.4.egg-info
command_not_found-0.3.egg-info
cryptography-3.4.8.egg-info
dbus_python-1.2.18.egg-info
distro-1.7.0.egg-info
distro_info-1.1+ubuntu0.1.egg-info
httplib2-0.20.2.egg-info
importlib_metadata-4.6.4.egg-info
jeepney-0.7.1.dist-info
keyring-23.5.0.egg-info
launchpadlib-1.10.16.egg-info
lazr.restfulclient-0.14.4.egg-info
lazr.uri-1.0.6.egg-info
more_itertools-8.10.0.egg-info
netifaces-0.11.0.egg-info
oauthlib-3.2.0.egg-info
pyparsing-2.4.7.egg-info
python_apt-2.4.0+ubuntu2.egg-info
setuptools-59.6.0.egg-info
six-1.16.0.egg-info
systemd_python-234.egg-info
ubuntu_advantage_tools-8001.egg-info
ufw-0.36.1.egg-info
unattended_upgrades-0.1.egg-info
wadllib-1.3.6.egg-info
zipp-1.0.0.egg-info

2. Venvs with items installed from sdists with older Pips:

Here cowsay 5.0 is sdist-only:

$ python3.10 -mvenv sdist-example.venv
$ sdist-example.venv/bin/pip -q install cowsay==5.0
$ ls sdist-example.venv/lib/python3.10/site-packages | grep -E "\.(dist-info|egg-info)" | sort
cowsay-5.0.egg-info
pip-22.0.2.dist-info
setuptools-59.6.0.dist-info

[jsirois]

Unfortunately, the Mac unit test errors are likely real; so this will have to wait until November to go green. The fix will likely be both hard won and utterly insignificant though; so it's safe to review this.

[jsirois]

Ok, sorted.

[benjyw]

Looks good.