boulder: Make boulder's own code (i.e. outside of external .yaml fiels) self-documentiong re. macros

[ermo]

Make it so that each version of boulder is fully self-documenting in terms of supported macros (actions and definitions), by ensuring that:

• macros have fields where they are defined that contain a non-empty "description" and "example" field (either being empty should be a build-time error).
• boulder recipe macros can output a human readable and a JSON formatted output of the above.

Some refactoring may be needed, but will ultimately make it so that the canonical documentation for what boulder supports is single-sourced from the boulder version itself.

The relevant source files for this are:

• src/package.rs
• src/build/job/phase.rs