introduce backend modules

src/backend/mod.rs

@@ -0,0 +1,14 @@
+mod pacman;
+
+use std::collections::HashSet;
+
+use crate::Package;
+
+pub use pacman::Pacman;
+
+pub trait Backend {
+    /// Get all packages that are installed in the system.
+    fn get_all_installed_packages() -> HashSet<Package>;
+    /// Get all packages that were installed in the system explicitly.
+    fn get_explicitly_installed_packages() -> HashSet<Package>;
+}

src/backend/pacman.rs

@@ -3,15 +3,9 @@ use std::collections::HashSet;
 use alpm::Alpm;
 use alpm::PackageReason::Explicit;
 
+use super::Backend;
 use crate::Package;
 
-pub trait Backend {
-    /// Get all packages that are installed in the system.
-    fn get_all_installed_packages() -> HashSet<Package>;
-    /// Get all packages that were installed in the system explicitly.
-    fn get_explicitly_installed_packages() -> HashSet<Package>;
-}
-
 pub struct Pacman;
 
 impl Backend for Pacman {