pub fn local_path(&self) -> Option<&'g Utf8Path> {
        match self {
            PackageSource::Path(path) | PackageSource::Workspace(path) => Some(path),
            _ => None,
        }
    }
Returns the local path if this is a local dependency, or `None` if it is an external
dependency.

The path is relative to the workspace root.
guppy/src/graph/graph_impl.rs