search: refactor scan_query callers to use split_query.

Refactor to use split_query instead of scan_query to split a query on
spaces. Preparation for refactoring scan_query into something smarter.

app/logical/post_query_builder.rb

@@ -760,6 +760,10 @@ class PostQueryBuilder
         list
       end
 
+      def split_query(query)
+        scan_query(query)
+      end
+
       def normalize_query(query, normalize_aliases: true, sort: true)
         tags = scan_query(query.to_s)
         tags = tags.map { |t| Tag.normalize_name(t) }