Generate transitive dependencies containing purls for specific ecosystems.

As an example, take the `wyvern` crate. This crate is packaged in nixpkgs using the `fetchCrate` function as its source. We can use the fact that `wyvern.src.url` contains crates.io to add a dependency on that crate.

We would have our normal component with the generic purl, and then have that depend on a component with a specific rust/crates.io purl.