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>;
+}