esm: fallback to readFileSync when source is nullish

When using the Modules Customization Hooks API to load CommonJS modules,
we want to support the returned value of `defaultLoad` which must be
nullish to preserve backward compatibility. This can be achieved by
fetching the source from the translator.

lib/internal/modules/esm/load.js

@@ -261,5 +261,6 @@ function throwUnknownModuleFormat(url, format) {
 module.exports = {
   defaultLoad,
   defaultLoadSync,
+  getSourceSync,
   throwUnknownModuleFormat,
 };

lib/internal/modules/esm/translators.js

@@ -58,6 +58,13 @@ const asyncESM = require('internal/process/esm_loader');
 const { emitWarningSync } = require('internal/process/warning');
 const { internalCompileFunction } = require('internal/vm');
 
+// Lazy-loading to avoid circular dependencies.
+let getSourceSync;
+function getSource(url) {
+  getSourceSync ??= require('internal/modules/esm/load').getSourceSync;
+  return getSourceSync(url).source;
+}
+
 /** @type {import('deps/cjs-module-lexer/lexer.js').parse} */
 let cjsParse;
 /**
@@ -276,7 +283,7 @@ function createCJSModuleWrap(url, source, isMain, loadCJS = loadCJSModule) {
   debug(`Translating CJSModule ${url}`);
 
   const filename = StringPrototypeStartsWith(url, 'file://') ? fileURLToPath(url) : url;
-  source = stringify(source);
+  source = stringify(source ?? getSource(new URL(url)));
 
   const { exportNames, module } = cjsPreparseModuleExports(filename, source);
   cjsCache.set(url, module);

test/es-module/test-esm-loader-hooks.mjs

@@ -746,4 +746,39 @@ describe('Loader hooks', { concurrency: true }, () => {
     assert.strictEqual(code, 0);
     assert.strictEqual(signal, null);
   });
+
+  it('should handle mixed of opt-in modules and non-opt-in ones', async () => {
+    const { code, signal, stdout, stderr } = await spawnPromisified(execPath, [
+      '--no-warnings',
+      '--experimental-loader',
+      `data:text/javascript,const fixtures=${JSON.stringify(fixtures.path('empty.js'))};export ${
+        encodeURIComponent(function resolve(s, c, n) {
+          if (s.endsWith('entry-point')) {
+            return {
+              shortCircuit: true,
+              url: 'file:///virtual-entry-point',
+              format: 'commonjs',
+            };
+          }
+          return n(s, c);
+        })
+      }export ${
+        encodeURIComponent(async function load(u, c, n) {
+          if (u === 'file:///virtual-entry-point') {
+            return {
+              shortCircuit: true,
+              source: `"use strict";require(${JSON.stringify(fixtures)});console.log("Hello");`,
+              format: 'commonjs',
+            };
+          }
+          return n(u, c);
+        })}`,
+      'entry-point',
+    ]);
+
+    assert.strictEqual(stderr, '');
+    assert.strictEqual(stdout, 'Hello\n');
+    assert.strictEqual(code, 0);
+    assert.strictEqual(signal, null);
+  });
 });