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.

lib/email/message_builder.rb

@@ -3,19 +3,6 @@
 # Builds a Mail::Message we can use for sending. Optionally supports using a template
 # for the body and subject
 module Email
-
-  module BuildEmailHelper
-    def build_email(*builder_args)
-      builder = Email::MessageBuilder.new(*builder_args)
-      headers(builder.header_args) if builder.header_args.present?
-      mail(builder.build_args).tap { |message|
-        if message && h = builder.html_part
-          message.html_part = h
-        end
-      }
-    end
-  end
-
   class MessageBuilder
     attr_reader :template_args