ComputeCriticalPath: Use topological sort to speed up function.

Use a topological sort to get a sorted list of edges to perform
the critical path weigth propagation as fast as possible.

The previous version used a data flow algorithm that forced
the values to be recomputed several times for far too many
edges on complex build plans.

For example, for a Fuchsia build plan with 93339 edges,
this reduces the ComputeCriticalPath metric from 1.9s to 80ms!

The unit-tests still pass, and manual testing shows that the
order of commands does not change before and after this change
for the example build plan above.

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