Get additional tool descriptor files relative to the main file

Descriptors can often include imports that refer to additional descriptors. This returns additional descriptors for the specified tool in the same or other directories that can be reached as a relative path. This endpoint can be useful for workflow engine implementations like cwltool to programmatically download all the descriptors for a tool and run it. This can optionally include other files described with FileWrappers such as test parameters and containerfiles.