add command to show the reasons a package is included in the build graph

[dhellmann]

For debugging, especially when there are conflicts, it would be useful to have a command to summarize the reasons a package appears in the build graph. It should show all paths from the top-level to all versions of the package, including all edges and the edge type.

Optional enhancements could be to:

• Filter by edge type
• Filter by package version
• Produce output in dot format suitable for rendering into a picture with graphviz