Function collect_drop_in_config_files 

fn collect_drop_in_config_files(
    dirs: &[impl AsRef<Path>],
) -> BTreeMap<Os, Vec<ConfigFile>>

Collects all valid drop-in config files from a list of input dirs.

First uses collect_drop_in_dirs to collect all drop-in directories from the top-level dirs. Then creates a map of target OSes and relevant file names (with specific file path locations). All masked drop-ins are ignored. Drop-in config files are sorted and read according to the UAPI config file specification.