added an in-memory cache for searching for existing beatmapsets inste…

…ad of looping every time the extraction directory which is slow.

OsuCollectionDownloader/Cache/FrozenDirectoryCache.cs

@@ -0,0 +1,15 @@
+using System;
+using System.Collections.Frozen;
+using System.Collections.Generic;
+using System.Collections.Immutable;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace OsuCollectionDownloader.Cache;
+
+internal sealed class FrozenDirectoryCache(string path)
+{
+    internal FrozenSet<string> Items { get; } = new HashSet<string>(Directory.EnumerateDirectories(path), StringComparer.OrdinalIgnoreCase).ToFrozenSet();
+}