fn to_layout_sources(
layout_paths: impl Iterator<Item = (ArrangementKind, String)>,
filesystem_path: Option<PathBuf>
) -> impl Iterator<Item = (ArrangementKind, DataSource)>
fn to_layout_sources(
layout_paths: impl Iterator<Item = (ArrangementKind, String)>,
filesystem_path: Option<PathBuf>
) -> impl Iterator<Item = (ArrangementKind, DataSource)>