feature: Support loading collections of duties through entrypoints

[pawamoy]

Is your feature request related to a problem? Please describe.

I'm copy-pasting (through templating) the same duties in every project. To reduce boilerplate (and noise in template updates), I imagine allowing collections of duties to be loaded from entrypoints.

Describe the solution you'd like

Allow discovering and loading duties from entrypoints. A published package exposes a module containing duties, and registers a "duty" entrypoint pointing to it. When running, duty can discover that module (through stdlib's entrypoint mechanisms) and load its duties, making them available to the final user. In the end, users can install packages to be able to run duties they provide.

Describe alternatives you've considered

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Additional context

See how that would integrate with auto-docs of duties (like in Griffe).

This could also mean a come-back of the "fulfill" project.

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