In addition of pruning the dependency graph based on the source file passed to us, we should support a per-workspace "allowlist" of sorts that allows people to specify exactly the crates that they'd like to be included in the graph. This seems like a useful feature if you care a lot about exhaustively finding all the references to a symbol in the workspace--you could allowlist the upstream crates you know depend on the symbol. I want to think carefully about the design--there's a lot we could do here, and I want the format/strategy we use to be extensible. Maybe a more general config file of some sort is warranted?

Whatever syntax we choose for this file, it should support both allowlisting and denylisting.