Replace igraph usage with networkx in project graph builder

[mwatts15]

The current project graph builder uses igraph for its graph data structure and utility library. igraph requires extra setup steps to run on Windows and, depending on the what's already installed, it requires extra steps on Linux as well. This is undesirable for documentation maintainers. networkx, on the other hand, is a relatively painless install.

This task is to replace the igraph code in build.py with networkx such that build.py still provides the needed input for visualization, but requires less effort for the initial setup.