[Proposal] Project Relationship Graphs

[darkowlzz]

This is a proposal for creating relationship graphs of a dependency project and its relationship with other projects. The dependency graphs could be used to visualize the relationships set by constraints and imports between the projects and packages.

This would be based on the Project Status work that's going on in #1367 . The project status data of a given project could be visualized by showing:

1. PROJECT IMPORTERS - projects that import the given project.
   

2. CONSTRAINTS set on the project by other projects
   

3. PACKAGE IMPORTERS - packages that import the given project and its packages.
   

Implementation of this would require modifications in cmd/dep/graphviz.go to generate dot outputs for the aforementioned data from project status.

Any corrections, additions and questions are welcome.