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.

examples/file-sharing.rs

@@ -219,8 +219,9 @@ mod network {
         ProtocolSupport, RequestId, RequestResponse, RequestResponseCodec, RequestResponseEvent,
         RequestResponseMessage, ResponseChannel,
     };
-    use libp2p::swarm::{ConnectionHandlerUpgrErr, SwarmBuilder, SwarmEvent};
-    use libp2p::{NetworkBehaviour, Swarm};
+    use libp2p::swarm::{
+        ConnectionHandlerUpgrErr, NetworkBehaviour, Swarm, SwarmBuilder, SwarmEvent,
+    };
     use std::collections::{hash_map, HashMap, HashSet};
     use std::iter;