Add :latest, :pkg, :pkg-latest aliases -- and decide what :latest tracks #2

Split out from #1 since it's worth its own discussion.

Normally :latest points to the "latest" image, but for a multi-version matrix build that's ambiguous. The open question: should :latest follow FreeBSD's default version Mk/bsd.default-versions.mk, or follow what the upstream Python image does with its latest tag?

Requests

• Add :latest, :pkg, and :pkg-latest aliases.
  • :pkg and :pkg-latest aliases should probably track the python3 package.
  • :latest is up for debate:
    • Upstream Python -- currently 3.14, or
    • FreeBSD default -- currently 3.11, moving to 3.12.

Personally, I lean for :latest to match upstream python, thoughts?