Use vcztools public API

[jeromekelleher]

We're using quite a few private functions from vcztools referencing their module locations. It would be good to limit to using symbols explicitly exported by the vcztools public API now:

https://github.com/sgkit-dev/vcztools/blob/main/vcztools/__init__.py

We can add the necessary things to the vcztools public API, so it would be good to have a list of the things that are needed.

[jeromekelleher]

(The idea is that we can keep track of the things we need to keep stable more easily this way)