Use the ecosystem convention for naming directories

https://github.com/pypa/distutils/pull/133 was recently merged, and it's great to see better support for alternative Python implementations, but unfortunately it was implemented in a way that is distinct from what I've seen in the rest of the community.

In particular the issue is the usage of sys.cache_tag for naming implementation directories. This works but is a deviation from the community norm of using sysconfig.get_config_var("VERSION"): for example see build_scripts.py from this repo and the uwsgi build system. This is workable for an alternative implementation as long as we set sys.cache_tag = sysconfig.get_config_var("VERSION"), which I guess isn't the end of the world, but I believe the correct resolution is to use the appropriate configuration flag.

I'd like to advocate for #107 being merged, which simply means replacing sys.cache_tag with sysconfig.get_config_var("VERSION") in #133.

Inspect the changes from merged PR #133 and the proposed resolution in PR #107, then compare the directory-naming convention in distutils/command/build_scripts.py. Confirm the relevant sysconfig value and update the naming path accordingly; done means alternative Python implementations use the ecosystem convention without requiring sys.cache_tag changes.