swarm-derive: Add prelude configuration option to NetworkBehaviour macro (#3055)

Currently, our `NetworkBehaviour` derive macro depends on the `libp2p` crate to be in scope. This prevents standalone usage which forces us to depend on `libp2p` in all our tests where we want to derive a `NetworkBehaviour`.

This PR introduces a `prelude` option that - by default - points to `libp2p::swarm::derive_prelude`, a new module added to `libp2p_swarm`. With this config option, users of `libp2p_swarm` can now refer to the macro without depending on `libp2p`, breaking the circular dependency in our workspace. For consistency with the ecosystem, the macro is now also re-exported by `libp2p_swarm` instead of `libp2p` at the same position as the trait that it implements.

Lastly, we introduce an off-by-default `macros` feature flag that shrinks the dependency tree for users that don't need the derive macro.

protocols/rendezvous/examples/discover.rs

@@ -143,7 +143,7 @@ impl From<Void> for MyEvent {
     }
 }
 
-#[derive(libp2p::NetworkBehaviour)]
+#[derive(libp2p::swarm::NetworkBehaviour)]
 #[behaviour(event_process = false)]
 #[behaviour(out_event = "MyEvent")]
 struct MyBehaviour {