Add an equivalent to github's "uses" directive

[sed-i]

Currently with spread we'd need to replicate a library of scripts across repos, or obtain the scripts from a central place (git repo, snap, etc.) as part of e.g. backend-prepare.

There are a few problems with this approach:

• Different groups of devs are unlikely to discover each other's helper scripts.
• Reusing existing scripts means the author of spread.yaml need to also install all the (implicit) deps of the scripts they are reusing.

It would be handy if:

1. spread users had guidelines on how to design recipes so they are usable by others;
2. spread had a place to register and discover reusable recipes.

References:

• Github's uses directive.
• pre-commit hooks, a centralized repo for scripts which depend on pystdlib and pre_commit_hooks.util only.
• A naive concept for reusable recipes using justfile.