Way to specify step dependencies but not include them in the input object

To help with reshaping inputs, it would be helpful to be able to specify input sources explicitly not propagated to the actual input or output object. This gives the user a way to inform the workflow checker that the input parameter isn't expected to match the signature of the underlying 'run', and doesn't pass through parameters to the underlying 'run' which may be problematic.

I believe it is the case that cwltool already does not pass through input object elements that do not match the signature, but this would make it formally specified.

- [ ] need to decide on the name

Example:

```
in:
render: { source: render, propagate: false }
source: {valueFrom: $(inputs.render.source)}
target: {valueFrom: $(inputs.render.target)}
renderlist: {valueFrom: $(inputs.render.renderlist)}

```

No repository files, tests, or entry points are named. Start by reviewing the CWL input-binding specification and the example in the issue; done means agreeing on the dependency-only syntax and formally specifying how those sources affect valueFrom evaluation without appearing in the underlying input or output object.

Written by the indexing model from the issue text.