* Changes directional diagram algorithm
to use depth-first-search (i.e. `nx.dfs_tree`)
to generate the directional edges.

* Removes private functions
`diagrams._collect_sources` and
`diagrams.get_directional_path_edges`

* Use networkx method to build the directional
diagram with reversed edges

* Move array-based edge flipping code into
the `return_edges=True` code path

* Changes directional diagram return type
for `return_edges=False` to a `nx.DiGraph`