Function build_file_records_with_reader 

fn build_file_records_with_reader<E, F>(
    files: &[String],
    config: &AtlasMapConfig,
    db_purposes: &BTreeMap<String, String>,
    read_text: &mut F,
) -> Result<(Vec<MapRecord>, Vec<String>, BTreeMap<String, Vec<String>>), E>
where
    E: From<AtlasMapError>,
    F: FnMut(&Path) -> Result<String, E>,

Build file purpose rows through a caller-owned bounded text reader.