module: deprecate Module._debug

The _debug of Module is undocumented and it useless here.

Author: JacksonTian

doc/api/deprecations.md

@@ -683,6 +683,15 @@ difference is that `querystring.parse()` does url encoding:
 { '%E5%A5%BD': '1' }
 ```
 
+<a id="DEP00XX"></a>
+### DEP00XX: Module.\_debug()
+
+Type: Runtime
+
+`Module._debug()` has been deprecated.
+
+*Note*: `Module._debug()` was never documented as an officially supported API.
+
 [`Buffer.allocUnsafeSlow(size)`]: buffer.html#buffer_class_method_buffer_allocunsafeslow_size
 [`Buffer.from(array)`]: buffer.html#buffer_class_method_buffer_from_array
 [`Buffer.from(buffer)`]: buffer.html#buffer_class_method_buffer_from_buffer

lib/module.js

@@ -80,11 +80,11 @@ Module.globalPaths = [];
 
 Module.wrapper = NativeModule.wrapper;
 Module.wrap = NativeModule.wrap;
-Module._debug = util.debuglog('module');
 
-// We use this alias for the preprocessor that filters it out
-const debug = Module._debug;
+const debug = util.debuglog('module');
 
+Module._debug = util.deprecate(debug, 'Module._debug is deprecated.',
+                               'DEP00XX');
 
 // given a module name, and a list of paths to test, returns the first
 // matching file in the following precedence.