move todo_per_backend into separate module

src/backend/mod.rs

@@ -1,6 +1,7 @@
 mod macros;
 mod pacman;
 mod rust;
+mod todo_per_backend;
 
 use std::collections::HashSet;
 use std::os::unix::process::CommandExt;
@@ -8,6 +9,8 @@ use std::process::Command;
 
 use crate::{Group, Package};
 
+pub(crate) use todo_per_backend::ToDoPerBackend;
+
 pub use pacman::Pacman;
 pub use rust::Rust;
 pub(in crate::backend) type Switches = &'static [&'static str];