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/components/plugin/filter_manager_spec.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require 'rails_helper'
-require_dependency 'plugin/filter_manager'
 
 describe Plugin::FilterManager do
   let(:instance) { Plugin::FilterManager.new }

spec/components/plugin/instance_spec.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require 'rails_helper'
-require_dependency 'plugin/instance'
 
 describe Plugin::Instance do
 

spec/components/plugin/metadata_spec.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require 'rails_helper'
-require_dependency 'plugin/metadata'
 
 describe Plugin::Metadata do
   context "parse" do