This is now handled in Context.prepare_fork().

This aligns the method name with has_open_grpc_channels().

Remove XFAIL mark from test_workspace_visible and remove the explicit
SourceCache.commit() in the workspace source plugin. Allow buildstream
to handle the commit logic.

Add handling for non-cached workspace sources in
`source.Source._generate_keys()`.

As a first step, add type hints to variables whose type `mypy` cannot
infer automatically. This is the minimal set of type hints that allow
running `mypy` without any arguments, and having it not fail.

We currently ignore C extensions that mypy can't process directly.
Later, we can look into generating stubs for such modules (potentially
automatically).

Adding the element full name and display key into all element related
messages removes the need to look up the plugintable via a plugin
unique_id just to retrieve the same values for logging and widget
frontend display. Relying on plugintable state is also incompatible
if the frontend will be running in a different process, as it will
exist in multiple states.

The element full name is now displayed instead of the unique_id,
such as in the debugging widget. It is also displayed in place of
'name' (i.e including any junction prepend) to be more informative.

Pave the way toward supporting the 'spawn' method of creating jobs, by
adding support for pickling ChildJobs. Introduce a new 'jobpickler'
module that provides an entrypoint for this functionality.

This also makes replays of jobs possible, which has made the debugging
of plugins much easier for me.

This variable is accessed multiple times per run and can be slow on
slow file systems.

We need to update every place where we were passing a yaml 'dict' to
now pass a 'MappingNode'

A 'clone' operation has an implicit understanding that it is expensive,
which is not the case of a 'copy' operation, which is more usually
a shallow copy.

Therefore renaming to 'clone'