[BuildStream] Create new containers-related and Python-related plugins repos

[Chiara Tolentino <ctolentino3891 gmail com>]

Proposal: Domain-specific repositories for container-related and
Python-related plugins
=======================================================================================

As discussed during the recent BuildStream Gathering, ideally
BuildStream core should remain small and 'general purpose' without
turning bst-external into a dumping ground/kitchen sink. The proposed
solution was to move domain-specific plugins out of core and/or
bst-external and keep them in separate repositories, each with their
own maintainer. These repositories would then have a clearer purpose
and the maintainer would be responsible for ensuring the stability and
reliability of these plugins.

Following this, I'd like to propose two new domain-specific
repositories: one for container-related plugins and another for
Python-related plugins.


Container plugins repository
----------------------------

This would contain the following plugins (which currently reside in
bst-external):
- Docker source [1]

And will eventually include such plugins as:
- Docker element
- OCI element [2][3]
- OCI source


Python plugins repository
-------------------------

This would contain the following plugins (which currently reside in
BuildStream core):
- pip source [4]
- pip element [5]
- distutils element [6]

In the future, this could be extended to include:
- sdist
- binary wheel


As this would introduce a breaking change, this should ideally be
merged before the release of BuildStream v1.4.

Please let me know your ideas regarding this proposal.

Cheers,
Chiara

[1]: https://gitlab.com/BuildStream/bst-external/blob/master/bst_external/sources/docker.py
[2]: https://gitlab.com/BuildStream/buildstream/merge_requests/655
[3]: https://gitlab.com/BuildStream/buildstream/issues/349
[4]: https://gitlab.com/BuildStream/buildstream/blob/master/buildstream/plugins/sources/pip.py
[5]: https://gitlab.com/BuildStream/buildstream/blob/master/buildstream/plugins/elements/pip.py
[6]: https://gitlab.com/BuildStream/buildstream/blob/master/buildstream/plugins/elements/distutils.py