Implement serpent macros piecemeal to make it easier to auto-port package.yml to stone.yaml format

[ermo]

As discussed in the meeting on 2024-07-06, we are going to aim to incrementally split out rc.yml into a format that is similar to how it's done in boulder url in the serpent tooling:

• actions
• definitions: base | x86_64

The driver for doing this incrementally is that for each time we split out a group, we want to set up associated CI/hooks that will lint packages NOT using the recommended macros/definitions and complain vociferously.

This will also serve to slowly familiarise people with the serpent concepts and ways of doing things over time.