Now all python packages that install python modules under
/usr/lib/pythonX.Y/site-packages will have a provider that indicates
their MAJOR (X) and MINOR (Y) versions.

pyX maps to the MAJOR version of the package, its objective is to allow
users to quickly install a python module without having to search around
for the correct package, doing `apk add py3.9:foo` will install whatever
packages provides the foo module.

The directories checked only go one level deep, so
'/usr/lib/python3.9/site-packages/date' will generate
py3:date=$pkgver-r$pkgrel.

files ending with .py also count and are added with their .py prefix
stripped away. so '/usr/lib/python3.9/site-packages/six.py' will
generate py3:six=$pkgver-r$pkgrel.

The reason for doing this is the same as creating pc:, so: and cmd:, it
is more reliable and robust to depened on what we known of what the package
provides than to try to guess via the pkgname.

Co-authored-by: Chloe Kudryavtsev <toast@toast.cafe>