Moved App Sources into separate files

lib/app_sources/fdroid.dart

@@ -0,0 +1,49 @@
+import 'package:html/parser.dart';
+import 'package:http/http.dart';
+import 'package:obtainium/providers/source_provider.dart';
+
+class FDroid implements AppSource {
+  @override
+  late String host = 'f-droid.org';
+
+  @override
+  String standardizeURL(String url) {
+    RegExp standardUrlRegEx = RegExp('^https?://$host/[^/]+/packages/[^/]+');
+    RegExpMatch? match = standardUrlRegEx.firstMatch(url.toLowerCase());
+    if (match == null) {
+      throw notValidURL;
+    }
+    return url.substring(0, match.end);
+  }
+
+  @override
+  Future<APKDetails> getLatestAPKDetails(String standardUrl) async {
+    Response res = await get(Uri.parse(standardUrl));
+    if (res.statusCode == 200) {
+      var latestReleaseDiv =
+          parse(res.body).querySelector('#latest.package-version');
+      var apkUrl = latestReleaseDiv
+          ?.querySelector('.package-version-download a')
+          ?.attributes['href'];
+      if (apkUrl == null) {
+        throw noAPKFound;
+      }
+      var version = latestReleaseDiv
+          ?.querySelector('.package-version-header b')
+          ?.innerHtml
+          .split(' ')
+          .last;
+      if (version == null) {
+        throw couldNotFindLatestVersion;
+      }
+      return APKDetails(version, [apkUrl]);
+    } else {
+      throw couldNotFindReleases;
+    }
+  }
+
+  @override
+  AppNames getAppNames(String standardUrl) {
+    return AppNames('F-Droid', Uri.parse(standardUrl).pathSegments.last);
+  }
+}