[refactor] Move files to core/ and servers/ subdirectories

CNProtocol, CNShared, CNStructs and Defines are now in core/.
CNLoginServer, CNShardServer and Monitor are now in servers/.

core/Core.hpp wraps all the core headers except for CNShared.hpp.

Defines.cpp has been renamed to Packets.cpp, and so has its
corresponding namespace, but not the header file. This is in preparation
for upcoming changes.

src/core/Core.hpp

@@ -0,0 +1,11 @@
+#pragma once
+
+/*
+ * Convenience header.
+ *
+ * We omit CNShared, as it's only relevant to the Login and Shard servers
+ * and the PlayerManager. We omit Defines, as CNProtocol already includes it.
+ */
+
+#include "core/CNProtocol.hpp"
+#include "core/CNStructs.hpp"