DEV: Upgrading Discourse to Zeitwerk (#8098)

Zeitwerk simplifies working with dependencies in dev and makes it easier reloading class chains. 

We no longer need to use Rails "require_dependency" anywhere and instead can just use standard 
Ruby patterns to require files.

This is a far reaching change and we expect some followups here.

spec/requests/admin/dashboard_controller_spec.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require 'rails_helper'
-require_dependency 'discourse_version_check'
 
 describe Admin::DashboardController do
   before do

spec/requests/admin/themes_controller_spec.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require 'rails_helper'
-require_dependency 'theme_serializer'
 
 describe Admin::ThemesController do
   fab!(:admin) { Fabricate(:admin) }

spec/requests/admin/versions_controller_spec.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require 'rails_helper'
-require_dependency 'version'
 
 describe Admin::VersionsController do
 

spec/requests/omniauth_callbacks_controller_spec.rb

@@ -582,8 +582,6 @@ RSpec.describe Users::OmniauthCallbacksController do
     end
 
     context 'after changing email' do
-      require_dependency 'email_updater'
-
       def login(identity)
         OmniAuth.config.mock_auth[:google_oauth2] = OmniAuth::AuthHash.new(
           provider: 'google_oauth2',