Evaluate PEX_ROOT cache structure.

[jsirois]

There are a few things to evaluate:

1. Versioning: Likely the cache should be versioned to allow clean incompatible changes to structure.
2. Hashing: cache keys have been built off sha1 content hashes since day1. Analyze attack vulnerability and see if sha256 could be used in (most) places. There are some places, maybe only in venvs, where path length becomes a problem due to shebang length limits.
3. Scavenging: What protions of the cache can be scavenged without affecting performance of X too badly. This is effectively about coming up with a pex3 cache tool that allows inspection and selective pruning.

[jsirois]

It turns out #1661 had to deal with 1 (versioning the installed_wheels portion of the cache and all the other portions that symlinked into it (venvs and unzipped_pexes)) and 2 (switching to sha256 hashes for installed_wheels) solved most of 1 in its particular case. Generally we'll still need cache versioning though. You can only change hash algorithms so often!

[jsirois]

I broke out #1662 for item 2 since that looks closeable.

[jsirois]

Item, 1 is now directly addressed with versioned cache entries like this:

pex/pex/cache/dirs.py

Lines 93 to 95 in 84a4196

	INSTALLED_WHEELS = Value(
	"installed_wheels",
	version=0,

Item 3 is mainly addressed with the pex3 cache {dir,info,purge} commands.