restructure project into a virtual manifest

- remove redundant readme
- de-duplicate Cargo.toml information using workspace inheritance
- rename "pacdef_core" to "pacdef"
- move "crates/main/main.rs" to "crates/pacdef/src/main.rs"

crates/pacdef/src/backend/macros.rs

@@ -0,0 +1,60 @@
+/// Used to implement parts of the `Backend` trait that should not change between the actual
+/// backends (boilerplate).
+macro_rules! impl_backend_constants {
+    () => {
+        fn get_binary_default(&self) -> Text {
+            BINARY
+        }
+
+        fn get_section(&self) -> Text {
+            SECTION
+        }
+
+        fn get_switches_info(&self) -> Switches {
+            SWITCHES_INFO
+        }
+
+        fn get_switches_install(&self) -> Switches {
+            SWITCHES_INSTALL
+        }
+
+        fn get_switches_noconfirm(&self) -> Switches {
+            SWITCHES_NOCONFIRM
+        }
+
+        fn get_switches_remove(&self) -> Switches {
+            SWITCHES_REMOVE
+        }
+
+        fn get_switches_make_dependency(&self) -> Switches {
+            SWITCHES_MAKE_DEPENDENCY
+        }
+
+        fn get_managed_packages(&self) -> &HashSet<Package> {
+            &self.packages
+        }
+
+        fn load(&mut self, groups: &HashSet<Group>) {
+            let own_section_name = self.get_section();
+
+            groups
+                .iter()
+                .flat_map(|g| &g.sections)
+                .filter(|section| section.name == own_section_name)
+                .flat_map(|section| &section.packages)
+                .for_each(|package| {
+                    self.packages.insert(package.clone());
+                })
+        }
+
+        fn as_any_mut(&mut self) -> &mut dyn std::any::Any {
+            self
+        }
+
+        fn supports_as_dependency(&self) -> bool {
+            SUPPORTS_AS_DEPENDENCY
+        }
+    };
+}
+
+pub(crate) use impl_backend_constants;