Function collect_spans_and_sources

pub(crate) fn collect_spans_and_sources(
    tcx: TyCtxt<'_>,
    krate: &Crate,
    src_root: &Path,
    include_sources: bool,
    generate_link_to_definition: bool,
) -> (FxIndexMap<PathBuf, String>, FxHashMap<Span, LinkFromSrc>)

This function will do at most two things:

1. Generate a span correspondence map which links an item span to its definition span.
2. Collect the source code files.

It returns the krate, the source code files and the span correspondence map.

Note about the span correspondence map: the keys are actually (lo, hi) of spans. We don’t need the span context later on, only their position, so instead of keep a whole Span, we only keep the lo and hi.