Expose tendril dependencies for graphing

You can see in the first commit, I was producing graphviz format from tendril.  I moved this out into api worker.    Exposing the dependency graph in a simple data format (json) makes more sense to start.
d'oh, yes... missed it on my second commit.

I actually have no idea what this stuff is for...  do I need a line here for "dependencies", my new method on tendril?