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/ipfs-private.rs

@@ -42,10 +42,10 @@ use libp2p::{
     multiaddr::Protocol,
     noise, ping,
     pnet::{PnetConfig, PreSharedKey},
-    swarm::SwarmEvent,
+    swarm::{NetworkBehaviour, SwarmEvent},
     tcp,
     yamux::YamuxConfig,
-    Multiaddr, NetworkBehaviour, PeerId, Swarm, Transport,
+    Multiaddr, PeerId, Swarm, Transport,
 };
 use std::{env, error::Error, fs, path::Path, str::FromStr, time::Duration};