refactor(lib): keep the actors inside the crate and mark the growing enums

Every release since 14 has been a major, because the actor mailboxes,
the share index's struct and the CLI's command enum were all exhaustive
public API: adding a peer message, a scanned field or a subcommand read
as a breaking change to the library. The actors, dispatcher, peer
plumbing and download store are how the client is built, not what a host
programs against, so they move inside the crate; PeerMessage, SharedFile
and the CLI command enums are non-exhaustive; ConnectionType is
re-exported at the root for the one thing hosts used it for. A one-time
break so that the next additions are not.

soulseek-rs-lib/tests/e2e.rs

@@ -24,10 +24,9 @@ use std::time::{Duration, Instant};
 
 use soulseek_rs::message::Message;
 use soulseek_rs::message::server::MessageFactory;
-use soulseek_rs::peer::ConnectionType;
 use soulseek_rs::{
-    Client, ClientSettings, ClientVersion, DownloadStatus, PeerAddress,
-    SessionLoss, UploadStatus,
+    Client, ClientSettings, ClientVersion, ConnectionType, DownloadStatus,
+    PeerAddress, SessionLoss, UploadStatus,
 };
 
 /// Only one test at a time may drive a server.