Use stricter check to skip prelude loading

Fixes #865

Author: st0012

lib/debug/prelude.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 return if ENV['RUBY_DEBUG_ENABLE'] == '0'
-return if defined?(::DEBUGGER__)
+return if defined?(::DEBUGGER__::Session)
 
 # Put the following line in your login script (e.g. ~/.bash_profile) with modified path:
 #

test/console/debugger_method_test.rb

@@ -208,5 +208,14 @@ def test_prelude_doesnt_override_debugger
         type "c"
       end
     end
+
+    def test_require_config_doesnt_cancel_prelude
+      run_ruby(program, options: "-Ilib -rdebug/config") do
+        assert_line_num(5)
+        type "a + b"
+        assert_line_text(/120/)
+        type "c"
+      end
+    end
   end
 end