Load only project config for InternalAffairs/UndefinedConfig

Going through the default config is fine for RuboCop itself but
extensions need to inject their config for RuboCop to pick them up.

This makes it necessary to load the extension, even if the extension itself
doesn't use it, like `rubocop-rails` or `rubocop-capybara`.

Instead just load config/default.yml from `Dir.pwd` in isolation and use just that.
This would make rubocop/rubocop-capybara#133 and rubocop/rubocop-rails@288c7ce unnecessary

I tested this against a bunch of extensions and the cop seems to continue its job.
It also fixes a false positive in `rubocop-factory_bot`

lib/rubocop/cop/internal_affairs/undefined_config.rb

@@ -5,12 +5,22 @@ module Cop
     module InternalAffairs
       # Looks for references to a cop configuration key that isn't defined in config/default.yml.
       class UndefinedConfig < Base
+        class << self
+          include FileFinder
+        end
+
         ALLOWED_CONFIGURATIONS = %w[
           Safe SafeAutoCorrect AutoCorrect Severity StyleGuide Details Reference Include Exclude
         ].freeze
         RESTRICT_ON_SEND = %i[[] fetch].freeze
         MSG = '`%<name>s` is not defined in the configuration for `%<cop>s` ' \
               'in `config/default.yml`.'
+        CONFIG_PATH = find_file_upwards('config/default.yml', Dir.pwd)
+        CONFIG = if File.exist?(CONFIG_PATH)
+                   ConfigLoader.load_yaml_configuration(CONFIG_PATH)
+                 else
+                   {}
+                 end
 
         # @!method cop_class_def(node)
         def_node_search :cop_class_def, <<~PATTERN
@@ -31,7 +41,7 @@ def on_new_investigation
           cop_class = cop_class_def(processed_source.ast).first
           return unless (@cop_class_name = extract_cop_name(cop_class))
 
-          @config_for_cop = RuboCop::ConfigLoader.default_configuration.for_cop(@cop_class_name)
+          @config_for_cop = CONFIG[@cop_class_name] || {}
         end
 
         def on_send(node)

spec/rubocop/cop/internal_affairs/undefined_config_spec.rb

@@ -12,9 +12,8 @@
         Defined: true
     YAML
 
-    allow(RuboCop::ConfigLoader).to receive(:default_configuration).and_return(
-      RuboCop::ConfigLoader.load_file('config/default.yml', check: false)
-    )
+    stub_const('RuboCop::Cop::InternalAffairs::UndefinedConfig::CONFIG',
+               RuboCop::ConfigLoader.load_yaml_configuration('config/default.yml'))
   end
 
   it 'does not register an offense for implicit configuration keys' do