pub fn workspace_path(&self) -> Option<&'g Utf8Path> {
        match self {
            PackageSource::Workspace(path) => Some(path),
            _ => None,
        }
    }
Returns the path if this is a workspace dependency, or `None` if this is a non-workspace
dependency.

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