[dagster] fix composition bug
This terminal_step_output jonx did not get handled correctly for the entirety of composites existence. We never tested the case where you have fully parallel subdags within a composite which is where it breaks down.
To handle correctly, we build up a dict of compute steps as we recurse and then use the mapping path to resolve through any output mappings to the actual compute step it comes from.
Test Plan: newly added test case
Reviewers: #ft, schrockn
Reviewed By: #ft, schrockn
Differential Revision: https://dagster.phacility.com/D847