sources: rename Sources::Strategies to Source::Extractor.

Rename Sources::Strategies to Source::Extractor. A Source::Extractor
represents a thing that extracts information from a given URL.

app/logical/source/extractor/null.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+module Source
+  class Extractor
+    class Null < Source::Extractor
+      def image_urls
+        [url]
+      end
+
+      def page_url
+        nil
+      end
+
+      def artists
+        ArtistFinder.find_artists(url)
+      end
+    end
+  end
+end