Function resolve_import 

pub fn resolve_import(
    base_dir: Option<&Path>,
    raw: &str,
) -> Result<PathBuf, String>

Resolve an import path.

• Absolute paths are returned as-is (normalized).
• Relative paths are resolved against base_dir.
• If the result is a directory, append /default.nix.

Errors

Returns an error if the path is relative but no base_dir is provided.