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/migration/column_dropper_spec.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require 'rails_helper'
-require_dependency 'migration/column_dropper'
 
 RSpec.describe Migration::ColumnDropper do
   def has_column?(table, column)

spec/components/migration/safe_migrate_spec.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require 'rails_helper'
-require_dependency 'migration/safe_migrate'
 
 describe Migration::SafeMigrate do
   before do

spec/components/migration/table_dropper_spec.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require 'rails_helper'
-require_dependency 'migration/table_dropper'
 
 describe Migration::TableDropper do