DEV: minor fixes related to zeitwerk

A couple of specs stopped working post zeitwerk. Adding missing require
corrected inheritance to avoid a circular ref.

app/controllers/tags_controller.rb

@@ -120,6 +120,8 @@ class TagsController < ::ApplicationController
   end
 
   def upload
+    require 'csv'
+
     guardian.ensure_can_admin_tags!
 
     file = params[:file] || params[:files].first

lib/import_export/category_structure_exporter.rb

@@ -3,7 +3,7 @@
 require "import_export/base_exporter"
 
 module ImportExport
-  class CategoryStructureExporter < ImportExport::BaseExporter
+  class CategoryStructureExporter < BaseExporter
 
     def initialize(include_group_users = false)
       @include_group_users = include_group_users