Dependencies not calculated properly for subtrees of noop tasks

[dobesv]

If a noop task depends on another noop task, the dependencies won't propagate from the inner task to the dependents of the outer task.

For example:

Task A: script
Task B: noop - depends on A
Task C: noop - depends on B
Task D: script - depends on C

If you run D, lage will forget there's a dependency between D and A and potentially run D before A.

I created a test case and proposed fix in this PR: #880