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'

- _composite -> __composite (internal)
- composite -> _composite (BuildStream private)
- composite_under -> _composite_under (BuildStream private)
- get -> _get (internal)

Users should not need to get access to any of those, and should only
need access to the ProvenanceInformation to print it.

This replaces the helper method by adding a 'get_provenance' on the node
directly

- Adapt all call sites
- Delay getting provenance wherever possible without major refactor

- adapt all call sites to use the new API

- Also take care of node_composite_move in the same way.
- Adapt all calling places