[BuildStream] Prepare for possible Python plugin migration in v1.6

[Chiara Tolentino <ctolentino3891 gmail com>]

(Forking off the discussion in [1] as this should be two separate
conversations really)

I agree that, as Tristan mentioned [2], the workload required for the
Python plugins
to move out of core is not feasible for v1.4, but it would probably be best to
at least start the conversation now about what would need to be discussed and
agreed upon to prepare to land this change in, hopefully, v1.6. For example:

1.) What process would we have for telling users in the future about moving
plugins out of core? Do we have to make a definitive list of plugins moving out
or a definitive list of plugins that are sure to remain in core? Do we make a
more vague warning? Do we need to make this announcement at all?

2.) As a starting point, what would everyone think about only keeping the
following plugins in core?
General elements:
- import
- junction
- compose
- script
- junction
- filter

Build elements:
- manual

Sources:
- local
- remote

3.) Is there anything else that we would need to start working on now to be
able to make a v1.6 deadline for this change?

It would be great to hear your thoughts on these.

Cheers,
Chiara

[1]: https://mail.gnome.org/archives/buildstream-list/2018-October/msg00057.html
[2]: https://mail.gnome.org/archives/buildstream-list/2018-October/msg00080.html