Teach Spectral to report a dependency graph

[nulltoken]

User story.
As a user of the VSCode plugin, when working on $reffed files that aren't root documents, Spectral properly find out the proper root file to lint.

Is your feature request related to a problem?
stoplightio/vscode-spectral#6 introduces a half-baked way to deal with this by managing an in-memory tree of dependencies based on the issues that have been in found. Although this helps (a bit) the "lint-while-you-work" experience, this is very fragile.

Describe the solution you'd like
Maybe a first step would be to have run()/runWithResolved() return an object representing the dep graph of the files that the resolver met. This dep graph may contain multiple roots. This object would also expose a helper function allowing one to find the proper root(s) from a random file.