build runner: proper threaded dependency management

After sorting the step stack so that dependencies can be popped before
their dependants are popped, there is still a situation left to handle
correctly:

Example:

A depends on:
  B
  C
D depends on:
  E
  F

They will be ordered like this:

A B C D E F

If there are 6+ cores, then all of them will be evaluated at once,
incorrectly evaluating A and D before their dependencies.

Starting evaluation of F and then E is correct, but waiting until they
are done is not correct because it should start working on B and C as
well.

This commit solves the problem by computing dependants in the dependency
loop checking logic, and then having workers queue up their dependants
when they finish their own work.

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