Add a minimal flatpak backend

Cargo.toml

@@ -33,3 +33,4 @@ strip = true
 default = []
 debian = ["pacdef_core/debian"]
 arch = ["pacdef_core/arch"]
+flatpak = ["pacdef_core/flatpak"]

crates/pacdef_core/Cargo.toml

@@ -35,3 +35,4 @@ rust-apt = { version = "0.5", optional = true }
 default = []
 arch = ["dep:alpm"]
 debian = ["dep:rust-apt"]
+flatpak = []

crates/pacdef_core/src/backend/actual/flatpak.rs

@@ -0,0 +1,50 @@
+use std::collections::HashSet;
+use std::fs::read_to_string;
+use std::path::PathBuf;
+use std::process::ExitStatus;
+
+use anyhow::{Context, Result};
+use serde_json::Value;
+
+use crate::backend::backend_trait::{Backend, Switches, Text};
+use crate::{impl_backend_constants, Group, Package};
+
+#[derive(Debug, Clone)]
+pub struct Flatpak {
+    pub(crate) packages: HashSet<Package>,
+}
+
+const BINARY: Text = "flatpak";
+const SECTION: Text = "flatpak";
+
+const SWITCHES_INSTALL: Switches = &["install"];
+const SWITCHES_INFO: Switches = &[];
+const SWITCHES_MAKE_DEPENDENCY: Switches = &[];
+const SWITCHES_NOCONFIRM: Switches = &[];
+const SWITCHES_REMOVE: Switches = &["uninstall"];
+
+const SUPPORTS_AS_DEPENDENCY: bool = false;
+
+impl Backend for Flatpak {
+    impl_backend_constants!();
+
+    fn get_all_installed_packages(&self) -> Result<HashSet<Package>> {
+        Ok(HashSet::new())
+    }
+
+    fn get_explicitly_installed_packages(&self) -> Result<HashSet<Package>> {
+        Ok(HashSet::new())
+    }
+
+    fn make_dependency(&self, _: &[Package]) -> Result<ExitStatus> {
+        panic!("not supported by {}", BINARY)
+    }
+}
+
+impl Flatpak {
+    pub(crate) fn new() -> Self {
+        Self {
+            packages: HashSet::new(),
+        }
+    }
+}

crates/pacdef_core/src/backend/actual/mod.rs

@@ -4,3 +4,5 @@ pub mod arch;
 pub mod debian;
 pub mod python;
 pub mod rust;
+#[cfg(feature = "flatpak")]
+pub mod flatpak;

crates/pacdef_core/src/backend/mod.rs

@@ -18,4 +18,6 @@ pub enum Backends {
     Debian,
     Python,
     Rust,
+    #[cfg(feature = "flatpak")]
+    Flatpak,
 }