This has bitrotted and does not work anymore, but before fixing that
I wanted to get a clean transfer of what we had before with only
import/formatting changes.

I haven't moved the mock for ButlerQuantumContext that these classes
assume, because I'll be modifying them to use the MockStorageClass and
MockDataset classes instead.

This seems to have been introduced by switching derivedComponents to
Mapping instead of dict, but that shouldn't have mattered.

This allows the instrument embedded in a pipeline to be passed into
the graph generation algorithm even when the pipeline is converted
to a sequence of TaskDef objects in advance.