For -fkeep-going do not duplicate dependency edge code --- contains !1799

We now compute the deps for -fkeep-going the same way that the original graph calculates them, so the edges are correct. Upsweep really ought to take the graph rather than a topological sort so we are never recalculating anything, but at least things are calculated consistently now.

TODO tests

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to Notes and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a Note and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• replace this message with a description motivating your change

If you have any questions don't hesitate to open your merge request and inquire in a comment. If your patch isn't quite done yet please do add prefix your MR title with WIP:.