Function parse_relationships 

pub fn parse_relationships(
    body: &str,
    section_start_line: usize,
    entity_names: &HashSet<&str>,
    default_sources: &[SourceEntry],
    errors: &mut Vec<ParseError>,
) -> Vec<Rel>

Parse relationships from the ## Relationships section body.

entity_names is the set of entity names defined in the file (for resolution). default_sources are the front matter sources used when no source: override.