Function validate_target_path_as_source_file 

fn validate_target_path_as_source_file(
    gctx: &GlobalContext,
    target_path: &Path,
    target_name: &str,
    target_kind: &TargetKind,
    unit_manifest_path: &Path,
    error_count: &mut usize,
) -> CargoResult<()>