Project jobset: Update DeclarativeJobsets to not use the project or jobsets columns on Bulids

[grahamc]

Extracted from #1093.

My methodology here is to:

1. Pick a commit from Builds: drop the jobset and project columns #1093
2. Check out the parent commit
3. Run the test suite and identify failing tests
4. Check out the picked commit and run those failing tests
5. Verify the picked commit fixed at least one test.

If it didn't, write a test and verify it is broken before / fixed after the picked commit.

In this case there was no failing test so I wrote one, and extracted the "get the jobset name from the jobset table" commit.