Fixed some crashes in response strings source generator, reorganized more submodules into their folders

src/NadekoBot/Modules/Nsfw/_Common/Downloaders/DanbooruImageDownloader.cs

@@ -0,0 +1,33 @@
+#nullable disable
+using System.Net.Http.Json;
+
+namespace NadekoBot.Modules.Nsfw.Common;
+
+public sealed class DanbooruImageDownloader : DapiImageDownloader
+{
+    // using them as concurrent hashsets, value doesn't matter
+    private static readonly ConcurrentDictionary<string, bool> _existentTags = new();
+    private static readonly ConcurrentDictionary<string, bool> _nonexistentTags = new();
+
+    public DanbooruImageDownloader(HttpClient http)
+        : base(Booru.Danbooru, http, "http://danbooru.donmai.us")
+    {
+    }
+
+    public override async Task<bool> IsTagValid(string tag, CancellationToken cancel = default)
+    {
+        if (_existentTags.ContainsKey(tag))
+            return true;
+
+        if (_nonexistentTags.ContainsKey(tag))
+            return false;
+
+        var tags = await _http.GetFromJsonAsync<DapiTag[]>(
+            _baseUrl + "/tags.json" + $"?search[name_or_alias_matches]={tag}",
+            _serializerOptions,
+            cancel);
+        if (tags is { Length: > 0 }) return _existentTags[tag] = true;
+
+        return _nonexistentTags[tag] = false;
+    }
+}