Switch task graph edges to direct dependencies

[gsoltis]

The main change in this PR switches the task graph construction to add edges only between direct dependencies, rather than the entire ancestor subtree.

As an example, say a depends on b, b depends on c. If b#build depends on c#some-task, and a#builddepends on^build`.

Currently, running a#build will do c#some-task, c#build, b#build, and a#build.
But, we don't need c#build. We just need c#some-task, since that's what b#build depends on, and a doesn't really know anything about c directly, just via b. So, we instead just draw an edge to direct dependencies, and then let them sort out their own dependencies.

For a large example, here are before and after graphs for running turbo run build in examples/kitchen-sink:

Before:

After:

Also get rid of an unused field, taskDeps, on Scheduler.

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