pub fn parse_external(&self) -> Option<ExternalSource<'g>> {
        match self {
            PackageSource::External(source) => ExternalSource::new(source),
            _ => None,
        }
    }
Attempts to parse an external source.

Returns `None` if the external dependency could not be recognized, or if it is a local
dependency.

For more about external sources, see the documentation for [`ExternalSource`](ExternalSource).
guppy/src/graph/graph_impl.rs