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/mod.rs

@@ -0,0 +1,26 @@
+mod actual;
+mod backend_trait;
+mod iter;
+mod macros;
+mod root;
+mod todo_per_backend;
+
+pub use backend_trait::Backend;
+pub use iter::BackendIter;
+pub use todo_per_backend::ToDoPerBackend;
+
+use pacdef_macros::Register;
+
+#[derive(Debug, Register)]
+pub enum Backends {
+    #[cfg(feature = "arch")]
+    Arch,
+    #[cfg(feature = "debian")]
+    Debian,
+    Flatpak,
+    Fedora,
+    Python,
+    Rust,
+    Rustup,
+    Void,
+}