In these revisions `::' dependency handling was simplified by not linking
the cohort nodes into the dependency graph. This broke dependency checking
on all but the first instance of a `::' target since all of the cohort nodes
now just form a collection of disconnected dependency graphs.
Fix this by keeping a back-reference in each cohort to its leader (the
first instance of a :: node with the same name) and a count of the number
of cohorts that need to be made before dependent nodes are scheduled.
Classically, we'd need six centurions for cohort, but in this case one
suffices...
e9b668c861