Add reverse dependency graph to builder depot

[eeyun]

As a Habitat engineer, In order to provide automated building functionality, I need to be able to access package reverse dependencies in the builder depot in order to automatically schedule builds.

The Story

This story is a spike about integrating the reverse dependency graph code into the builder depot.

Criteria of acceptance

1. The builder depot is able to build the dependency graph on startup
2. The builder depot logs the graph stats to console at startup.

[reset]

This has taken form in builder-core and builder-scheduler crates