Function module_key 

pub fn module_key(
    path: &str,
    module_roots: &[String],
    module_depth: usize,
) -> String

Compute a module key from an input path.

Rules:

• Root-level files become "(root)".
• If the first directory segment is in module_roots, include up to module_depth directory segments.
• Otherwise, the module key is the first directory segment.