Pipeline package manager

It's a follow-up to the discussion to FOSDEM: https://docs.google.com/document/d/1yK2J_sv1ceLYYbeLzEW8VZ3HEr8AxZcZr8GhMk6gLUY/edit#heading=h.wif2w2yslznh

It would be great to have a library manager for Pipeline.
The following features would be useful:
1) Ability to develop, store and version Pipeline packages in SCM (e.g. Git)
2) Ability to share these libraries in open-source with minimal efforts from the developer and user sides
3) require() step, which loads libs and its dependencies
4) version conflict prevention engine (API deprecation, etc.)
5) Pipeline documentation visualization for packages in Pipeline editor
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