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/distributed-key-value-store.rs

@@ -50,8 +50,8 @@ use libp2p::kad::{
 use libp2p::{
     development_transport, identity,
     mdns::{Mdns, MdnsConfig, MdnsEvent},
-    swarm::SwarmEvent,
-    NetworkBehaviour, PeerId, Swarm,
+    swarm::{NetworkBehaviour, SwarmEvent},
+    PeerId, Swarm,
 };
 use std::error::Error;