Instead of treating the trigger as the "main" service and the
triggered service as subsidary, now we treat the triggered
service as the service and the trigger as "subsidary". This
needs some special handling when we work out which services
go in the default bundle, but it works better for declaring
dependencies on triggered services because it means the
dependency runs after the triggered service comes up, not
just when the watcher-for-events starts
Good: this means it's not hanging holding the s6 dataase lock.
Bad: it's the ugliest implementation and doesn't deserve to be preserved
(tbf the ugliness is not new)
