Function relativize_level 

pub fn relativize_level(
    level: &mut LevelGraph,
    target: &Path,
    roots: &BTreeMap<String, String>,
)

Rewrite a file-based level graph from absolute paths to relativized ids:

• file node ids (absolute paths) → {target}/rel or {root}/rel;
• edge endpoints, node parents and cycle node lists follow the same map;
• external node path attributes are relativized too;
• redundant/empty path attributes are dropped (a file node’s id IS its path, so it carries none).