expose PANTS_VERSION and make it string-comparable

[tgolsson]

Proposed fix for #18528.

Very "dumb" implementation exposing it as a constant to all BUILD files. Unfortunately packaging.version.Version cannot be compared to strings, so I'm shimming it with a local Version class that allows this. I could do this in a more localized manner as this'll affect all uses of PANTS_SEMVER - but it seems like it'll reduce boilerplate in a lot of places. Happy to change based on opinion!

We could also use an explicit comparator, or expose Version as well:

if pants_version_after("2.13"):

or

if PANTS_VERSION >= Version("2.13")

[kaos]

🎉