move todo_per_backend into separate module

src/backend/todo_per_backend.rs

@@ -0,0 +1,48 @@
+use super::Backend;
+use crate::Package;
+
+pub(crate) struct ToDoPerBackend(Vec<(Box<dyn Backend>, Vec<Package>)>);
+
+impl ToDoPerBackend {
+    pub(crate) fn new() -> Self {
+        Self(vec![])
+    }
+
+    pub(crate) fn push(&mut self, item: (Box<dyn Backend>, Vec<Package>)) {
+        self.0.push(item);
+    }
+
+    pub(crate) fn into_iter(self) -> impl Iterator<Item = (Box<dyn Backend>, Vec<Package>)> {
+        self.0.into_iter()
+    }
+
+    pub(crate) fn iter(&self) -> impl Iterator<Item = &(Box<dyn Backend>, Vec<Package>)> {
+        self.0.iter()
+    }
+
+    pub(crate) fn nothing_to_do_for_all_backends(&self) -> bool {
+        self.0.iter().all(|(_, diff)| diff.is_empty())
+    }
+
+    pub(crate) fn install_missing_packages(&self) {
+        self.0
+            .iter()
+            .for_each(|(backend, diff)| backend.install_packages(diff));
+    }
+
+    pub(crate) fn is_empty(&self) -> bool {
+        self.0.iter().all(|(_, packages)| packages.is_empty())
+    }
+
+    pub(crate) fn show(&self) {
+        for (backend, packages) in self.iter() {
+            if packages.is_empty() {
+                continue;
+            }
+            println!("{}", backend.get_section());
+            for package in packages {
+                println!("  {package}");
+            }
+        }
+    }
+}