feat(core): conditionally support folder based discovery of ESM

By setting `MIKRO_ORM_DYNAMIC_IMPORTS` we now get a dynamic import even
tho we still compile to CommonJS. This enabled support for folder based
discovery when using ES modules.

Related: #2631

packages/core/src/metadata/MetadataDiscovery.ts

@@ -959,7 +959,7 @@ export class MetadataDiscovery {
 
   private async initEnumValues(prop: EntityProperty, path: string): Promise<void> {
     path = Utils.normalizePath(this.config.get('baseDir'), path);
-    const exports = await import(path);
+    const exports = await Utils.dynamicImport(path);
     const target = exports[prop.type] || exports.default;
 
     if (target) {
@@ -994,7 +994,7 @@ export class MetadataDiscovery {
   }
 
   private async getEntityClassOrSchema(path: string, name: string) {
-    const exports = await import(path);
+    const exports = await Utils.dynamicImport(path);
     const targets = Object.values<Dictionary>(exports)
       .filter(item => item instanceof EntitySchema || (item instanceof Function && MetadataStorage.isKnownEntity(item.name)));
 

packages/core/src/utils/ConfigurationLoader.ts

@@ -21,7 +21,7 @@ export class ConfigurationLoader {
       path = Utils.normalizePath(path);
 
       if (await pathExists(path)) {
-        const config = await import(path);
+        const config = await Utils.dynamicImport(path);
         /* istanbul ignore next */
         let tmp = config.default ?? config;
 

packages/core/src/utils/Utils.ts

@@ -752,6 +752,20 @@ export class Utils {
     return createRequire(resolve(from))(id);
   }
 
+  /**
+   * Hack to keep dynamic imports even when compiling to CJS.
+   * We can't use it always, as it would break ts-node.
+   * @see https://github.com/microsoft/TypeScript/issues/43329#issuecomment-922544562
+   */
+  static async dynamicImport<T = any>(id: string): Promise<T> {
+    if (!process.env.MIKRO_ORM_DYNAMIC_IMPORTS) {
+      return import(id);
+    }
+
+    /* istanbul ignore next */
+    return Function(`return import('${id}')`)();
+  }
+
   static getORMVersion(): string {
     /* istanbul ignore next */
     try {

tests/EntityManager.mysql.test.ts

@@ -13,7 +13,6 @@ import { Author2Subscriber } from './subscribers/Author2Subscriber';
 import { EverythingSubscriber } from './subscribers/EverythingSubscriber';
 import { FlushSubscriber } from './subscribers/FlushSubscriber';
 import { Test2Subscriber } from './subscribers/Test2Subscriber';
-import { ManualAuthor2Subscriber } from './subscribers/ManualAuthor2Subscriber';
 
 describe('EntityManagerMySql', () => {