esm: resolve main from package.json when importing from directory

lib/internal/modules/esm/resolve.js

@@ -587,12 +587,35 @@ function shouldBeTreatedAsRelativeOrAbsolutePath(specifier) {
   return false;
 }
 
+function resolveRelativePackage(specifier /* string */, base /* URL */) {
+  const resolved = new URL(specifier, base);
+  const stat = tryStatSync(resolved);
+  if (stat.isDirectory()) {
+    let packageJSONPath;
+    if (!StringPrototypeEndsWith(resolved.pathname, '/')) {
+      packageJSONPath = `${resolved.pathname}/package.json`;
+    } else {
+      packageJSONPath = `${resolved.pathname}package.json`;
+    }
+    const packageJSONUrl = new URL(packageJSONPath, base);
+    packageJSONPath = fileURLToPath(packageJSONUrl);
+    const packageConfig = getPackageConfig(packageJSONPath, base);
+    return packageMainResolve(packageJSONUrl, packageConfig, base);
+  }
+  return new URL(specifier, base);
+}
+
 function moduleResolve(specifier /* string */, base /* URL */) { /* -> URL */
   // Order swapped from spec for minor perf gain.
   // Ok since relative URLs cannot parse as URLs.
   let resolved;
   if (shouldBeTreatedAsRelativeOrAbsolutePath(specifier)) {
-    resolved = new URL(specifier, base);
+    if (getOptionValue('--experimental-specifier-resolution') === 'node') {
+      // experimental-specifier-resolution also allows importing from directory
+      resolved = resolveRelativePackage(specifier, base);
+    } else {
+      resolved = new URL(specifier, base);
+    }
   } else {
     try {
       resolved = new URL(specifier);

test/es-module/test-esm-specifiers.mjs

@@ -9,12 +9,18 @@ import { fileURLToPath } from 'url';
 import commonjs from '../fixtures/es-module-specifiers/package-type-commonjs';
 // esm index.js
 import module from '../fixtures/es-module-specifiers/package-type-module';
+// esm main.js
+import pkg_main from '../fixtures/es-module-specifiers/package-with-main';
+// esm exports_main.js
+import pkg_export from '../fixtures/es-module-specifiers/package-with-exports';
 // Notice the trailing slash
 import success, { explicit, implicit, implicitModule, getImplicitCommonjs }
   from '../fixtures/es-module-specifiers/';
 
 assert.strictEqual(commonjs, 'commonjs');
 assert.strictEqual(module, 'module');
+assert.strictEqual(pkg_main, 'package-with-main');
+assert.strictEqual(pkg_export, 'package-with-exports');
 assert.strictEqual(success, 'success');
 assert.strictEqual(explicit, 'esm');
 assert.strictEqual(implicit, 'cjs');

test/fixtures/es-module-specifiers/package-with-exports/exports_main.js

@@ -0,0 +1,3 @@
+const identifier = 'package-with-exports';
+console.log(identifier);
+module.exports = identifier;

test/fixtures/es-module-specifiers/package-with-exports/package.json

@@ -0,0 +1,6 @@
+{
+  "main": "main.js",
+  "exports": {
+    ".": "./exports_main.js"
+  }
+}

test/fixtures/es-module-specifiers/package-with-main/main.js

@@ -0,0 +1,3 @@
+const identifier = 'package-with-main';
+console.log(identifier);
+module.exports = identifier;

test/fixtures/es-module-specifiers/package-with-main/package.json

@@ -0,0 +1,3 @@
+{
+  "main": "main.js"
+}