Two targets can swap positions with pantsd (#7583)

[illicitonion]

Before this PR, nothing would remove the edges of a dirty node, so if
two nodes swapped positions in the graph (e.g. if a dependency between
two targets inverted), a cycle would be detected.

With this PR, if we detect a cycle, but detect that there may be dirty
edges in play, we fully clear that node (including removing its edges),
which will cause it being re-triggered from scratch.

This is specifically in place to handle the cycle scenario - the dirty
bit, and dependency Generations are still the primary mechanism for
handling re-use of old versions.

There's an ugliness here that we still don't remove obsolete edges, so
if Generation 2 of a node has differing dependencies from Generation 1,
the dependency from Generation 1 will still dirty Generation 2. We may
want to consider solving that separately as/when it becomes a
significant issue, or we may want to re-work this PR to do something
like that... This PR happens to cover a part of that problem, but only
where it causes definitive problems (a fake cycle) rather than also
where it causes performance problems.

There's probably a slightly more principled solution here along the
lines of:

• Rather than using () as an edge weight in the graph, use the
  Generation of the dependee Node as an edge weight.
• When doing cycle detection, compare the edge weight against the
  generation of the node, and ignore obsolete edges.
  but I would want to think about that a lot more before doing it...

[stuhood]

Thanks!