Revert "module: fix inconsistency between load and _findPath"

This reverts commit 1b92214
from PR #22382.

See the discussion at
nodejs/citgm#604

PR-URL: #23228
Refs: #22382
Fixes: #4778
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

lib/internal/modules/cjs/loader.js

@@ -218,16 +218,6 @@ function tryExtensions(p, exts, isMain) {
   return false;
 }
 
-function readExtensions() {
-  const exts = Object.keys(Module._extensions);
-  for (var i = 0, j = 0; i < exts.length; ++i) {
-    if (path.extname(exts[i]) === '')
-      exts[j++] = exts[i];
-  }
-  exts.length = j;
-  return exts;
-}
-
 var warned = false;
 Module._findPath = function(request, paths, isMain) {
   if (path.isAbsolute(request)) {
@@ -284,15 +274,15 @@ Module._findPath = function(request, paths, isMain) {
       if (!filename) {
         // try it with each of the extensions
         if (exts === undefined)
-          exts = readExtensions();
+          exts = Object.keys(Module._extensions);
         filename = tryExtensions(basePath, exts, isMain);
       }
     }
 
     if (!filename && rc === 1) {  // Directory.
       // try it with each of the extensions at "index"
       if (exts === undefined)
-        exts = readExtensions();
+        exts = Object.keys(Module._extensions);
       filename = tryPackage(basePath, exts, isMain);
       if (!filename) {
         filename = tryExtensions(path.resolve(basePath, 'index'), exts, isMain);

test/known_issues/test-module-deleted-extensions.js

@@ -0,0 +1,18 @@
+'use strict';
+// Refs: https://github.com/nodejs/node/issues/4778
+const common = require('../common');
+const assert = require('assert');
+const fs = require('fs');
+const path = require('path');
+const tmpdir = require('../common/tmpdir');
+const file = path.join(tmpdir.path, 'test-extensions.foo.bar');
+
+tmpdir.refresh();
+fs.writeFileSync(file, '', 'utf8');
+require.extensions['.foo.bar'] = (module, path) => {};
+delete require.extensions['.foo.bar'];
+require.extensions['.bar'] = common.mustCall((module, path) => {
+  assert.strictEqual(module.id, file);
+  assert.strictEqual(path, file);
+});
+require(path.join(tmpdir.path, 'test-extensions'));