Function build_base_index 

pub fn build_base_index(
    overrides: &HashMap<String, String>,
) -> HashMap<String, String>

Builds a base-name lookup index from type override keys.

Each key is normalized to its last path segment (e.g. "chrono::DateTime" → "DateTime"). This index is used as a fallback when an exact full-path match fails, which happens when the Rust source uses an imported name rather than a fully-qualified path.