Improve filter settings migration with own source plugin.

migrations/d5_filter_format.yml

@@ -3,7 +3,7 @@ label: Drupal 5 filter format configuration
 migration_tags:
   - Drupal 5
 source:
-  plugin: d6_filter_format
+  plugin: d5_filter_format
 process:
   format:
     -
@@ -24,11 +24,13 @@ process:
         bypass: true
         map:
           filter:
-            - filter_html
-            - filter_html_escape
-            - filter_autop
-            - filter_url
-      settings: settings
+            0: filter_html
+            1: filter_html_escape
+            2: filter_autop
+            3: filter_url
+      settings:
+        plugin: filter_settings
+        source: settings
       weight: weight
       status:
         plugin: default_value