Always return a mutable Int2ObjectSortedMap from Group.unpack()

Originally I used a singleton map if possible, to reduce overhead in the
common case of a group containing a single file. However, the
work-in-progress Cache class needs to mutate the map.

Eventually my plan is to replace the Int2ObjectSortedMaps in Js5Index
and Group with arrays, solving both the overhead and mutation problems.

Signed-off-by: Graham <gpe@openrs2.org>

cache/src/main/kotlin/org/openrs2/cache/Group.kt

@@ -3,7 +3,6 @@ package org.openrs2.cache
 import io.netty.buffer.ByteBuf
 import it.unimi.dsi.fastutil.ints.Int2ObjectAVLTreeMap
 import it.unimi.dsi.fastutil.ints.Int2ObjectSortedMap
-import it.unimi.dsi.fastutil.ints.Int2ObjectSortedMaps
 import org.openrs2.buffer.use
 
 public object Group {
@@ -12,7 +11,9 @@ public object Group {
 
         val singleEntry = group.singleOrNull()
         if (singleEntry != null) {
-            return Int2ObjectSortedMaps.singleton(singleEntry.id, input.retain())
+            val files = Int2ObjectAVLTreeMap<ByteBuf>()
+            files[singleEntry.id] = input.retain()
+            return files
         }
 
         require(input.isReadable)