Sketch out `OmniBorGraph` type.

[alilleybrinker]

An OmniBor is the type of a single record (as found in a binary or in an associated file). We should also have a type (perhaps, OmniBorGraph?) that represents the full expansion of the dependency graph of inputs, as a Directed-Acyclic Graph. We'd probably want to use the petgraph crate for this.

[alilleybrinker]

With the recent reworking of the spec, this would now be the Artifact Dependency Graph, so ArtifactDependencyGraph in the code.

A bit of a long name, perhaps consider shortening? ArtifactDepGraph?

[alilleybrinker]

Same as #66, this can be closed.