Generate dot file

Low prio.

Nice to have: since we know all the dependencies, we can also generate a dot-file to create a gigantic graph using [graphviz](https://www.graphviz.org/). We can use that when visualizing or explaining things.

Typically straight-forward to generate (emit all the nodes and edges, the rest happens automatically), but not clear how useful it would be though, large graphs tend to be ... large.

No file, test, or entry point is named. First locate the dependency traversal and existing visualization or serialization entry points, then determine how Graphviz DOT output should represent the known dependencies. Done means emitting all dependency nodes and edges as valid DOT output, with the usefulness of large graphs considered.

Written by the indexing model from the issue text.