Commands or tools for viewing build dependencies

[larsbergstrom]

We have two scenarios here that are tough for us on Servo today:

• Understanding why X is rebuilding in a given cargo build invocation (e.g., why is script rebuilding?)
• Seeing what the dependency chain is for a given crate (e.g., what all is depended upon by script?)

To date, we've used the fingerprint logging support, but that's somewhat awkward at best. It would be awesome to have some kinda of tool or debugging infra to make this better.

cc @pcwalton @metajack @glennw