Topological sort of the dependency graph

A top sort (a tree-like layout) of the dependency graph would be helpful. This enhancement would help users understand how the modules are structured.

I say tree-like because,
1. the modules with no dependencies will form the root(s).
2. cycles may be formed as well.

No files, tests, or entry points are named. Start by locating the dependency-graph construction and visualization entry points, then review how roots and cycles are represented. Done requires an agreed tree-like layout for multiple roots and cyclic graphs, with corresponding test coverage.

Written by the indexing model from the issue text.