Do not use inherited constants to fix NoMethodError

There was an error with Ruby `const_get` behaviour and the load order
of the ruby files which was causing the application logic to use the
wrong class. This commit fixes this issue.

app/finders/issues_finder.rb

@@ -41,7 +41,7 @@ class IssuesFinder < IssuableFinder
   # rubocop: enable CodeReuse/ActiveRecord
 
   def params_class
-    IssuesFinder::Params
+    IssuesFinder.const_get(:Params, false)
   end
 
   # rubocop: disable CodeReuse/ActiveRecord