Later when we implement the ability to continue processing when tasks have
failed to compile, we will also need to avoid processing tasks that depend
upon the failed task.
This refactoring exposes this list of dependent tasks in a way that can be
used to skip processing of tasks that depend upon a failed task.
It also changes the blocking model of the parallel mode of operation so
that non-typings tasks are now blocked on their corresponding typings task.
Previously the non-typings tasks could be triggered to run in parallel to
the typings task, since they do not have a hard dependency on each other,
but this made it difficult to skip task correctly if the typings task failed,
since it was possible that a non-typings task was already in flight when
the typings task failed. The result of this is a small potential degradation
of performance in async parallel processing mode, in the rare cases that
there were not enough unblocked tasks to make use of all the available
workers.
PR Close#36083
Pete Bacon Darwin