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

src/NadekoBot/Modules/Nsfw/_Common/Downloaders/E621ImageDownloader.cs

@@ -0,0 +1,32 @@
+#nullable disable
+using System.Net.Http.Json;
+
+namespace NadekoBot.Modules.Nsfw.Common;
+
+public class E621ImageDownloader : ImageDownloader<E621Object>
+{
+    public E621ImageDownloader(HttpClient http)
+        : base(Booru.E621, http)
+    {
+    }
+
+    public override async Task<List<E621Object>> DownloadImagesAsync(
+        string[] tags,
+        int page,
+        bool isExplicit = false,
+        CancellationToken cancel = default)
+    {
+        var tagString = ImageDownloaderHelper.GetTagString(tags, isExplicit);
+        var uri = $"https://e621.net/posts.json?limit=32&tags={tagString}&page={page}";
+        using var req = new HttpRequestMessage(HttpMethod.Get, uri);
+        req.Headers.AddFakeHeaders();
+        using var res = await _http.SendAsync(req, cancel);
+        res.EnsureSuccessStatusCode();
+
+        var data = await res.Content.ReadFromJsonAsync<E621Response>(_serializerOptions, cancel);
+        if (data?.Posts is null)
+            return new();
+
+        return data.Posts.Where(x => !string.IsNullOrWhiteSpace(x.File?.Url)).ToList();
+    }
+}