Function relation_source 

fn relation_source<'a>(
    owners: &GraphOwners,
    entities: &'a BTreeMap<String, GraphEntity>,
    graph: &SymbolGraph,
    symbol_index: &GraphSymbolIndex<'_>,
    relation: &SymbolRelation,
    control: &IndexWorkControl,
) -> Result<&'a GraphEntity, CliError>

Resolve one parser relation’s unique local source entity when possible.