deps: Dependency tree isn't rebuilt if a dependency's dependency tree changes

[obiwac]

If X depends on Y which depends on Z, if I add dependency W to Y, X's dependency tree isn't currently updates.

I'm not sure what the solution here is other than forgoing dependency tree hashes completely (and thus have to traverse the entire dependency tree with dep-tree on each build). Maybe we could just make no guarantees and provide an easy way (other than bob clean to rebuild the dependency tree.