Return MappingProxyType instead of copying after each return.

This reverts commit af54ac1.

Following a [suggested change][suggested-change], this moves to using
immutable `MappingProxyType` as return values, bypassing the need for
copying.

[suggested-change](#116 (review))

mcproto/packets/packet_map.py

@@ -4,11 +4,10 @@
 import pkgutil
 from collections.abc import Iterator, Mapping, Sequence
 from functools import lru_cache
-from types import ModuleType
+from types import MappingProxyType, ModuleType
 from typing import Literal, NamedTuple, NoReturn, overload
 
 from mcproto.packets.packet import ClientBoundPacket, GameState, Packet, PacketDirection, ServerBoundPacket
-from mcproto.utils.decorators import copied_return
 
 __all__ = ["generate_packet_map"]
 
@@ -95,9 +94,8 @@ def generate_packet_map(
     ...
 
 
-@copied_return
 @lru_cache()
-def generate_packet_map(direction: PacketDirection, state: GameState) -> dict[int, type[Packet]]:
+def generate_packet_map(direction: PacketDirection, state: GameState) -> Mapping[int, type[Packet]]:
     """Dynamically generated a packet map for given ``direction`` and ``state``.
 
     This generation is done by dynamically importing all of the modules containing these packets,
@@ -124,4 +122,4 @@ def generate_packet_map(direction: PacketDirection, state: GameState) -> dict[in
             if issubclass(packet_class, direction_class):
                 packet_map[packet_class.PACKET_ID] = packet_class
 
-    return packet_map
+    return MappingProxyType(packet_map)