Function index_references 

pub fn index_references(
    graph: &ModuleGraph,
    sources: &HashMap<PathBuf, ParsedModuleSource>,
    unsaved: &HashSet<PathBuf>,
) -> ReferenceIndex

Walk sources and record every identifier that ModuleGraph::definition_of can resolve.

unsaved names files whose text came from an editor buffer. The index reports that so a consumer knows whether it is looking at the file it just edited.