esm: leverage loaders when resolving subsequent loaders

dev_fixtures_do_not_merge/index.mjs

@@ -0,0 +1,3 @@
+import symbol from 'yyy/lib.mjs';
+
+console.log(`main`, symbol);

dev_fixtures_do_not_merge/lib.mjs

@@ -0,0 +1,3 @@
+console.log(`lib`);
+
+export default 42;

dev_fixtures_do_not_merge/loader-a.mjs

@@ -0,0 +1,4 @@
+export async function resolve(specifier, context, defaultResolve) {
+  console.log(`loader-a`, {specifier});
+  return defaultResolve(specifier.replace(/^xxx\//, `./`));
+}

dev_fixtures_do_not_merge/loader-b.mjs

@@ -0,0 +1,4 @@
+export async function resolve(specifier, context, defaultResolve) {
+  console.log(`loader-b`, {specifier});
+  return defaultResolve(specifier.replace(/^yyy\//, `./`));
+}

lib/internal/process/esm_loader.js

@@ -60,17 +60,24 @@ async function initializeLoader() {
     cwd = 'file:///';
   }
 
+  const keyedExportsList = [];
+
   // A separate loader instance is necessary to avoid cross-contamination
   // between internal Node.js and userland. For example, a module with internal
   // state (such as a counter) should be independent.
   const internalEsmLoader = new ESMLoader();
 
-  // Importation must be handled by internal loader to avoid poluting userland
-  const keyedExportsList = await internalEsmLoader.import(
-    customLoaders,
-    pathToFileURL(cwd).href,
-    ObjectCreate(null),
-  );
+  for (const customLoader of customLoaders) {
+    // Importation must be handled by internal loader to avoid poluting userland
+    const keyedExportsSublist = await internalEsmLoader.import(
+      [customLoader],
+      pathToFileURL(cwd).href,
+      ObjectCreate(null),
+    );
+
+    await internalEsmLoader.addCustomLoaders(keyedExportsSublist);
+    keyedExportsList.push(...keyedExportsSublist);
+  }
 
   // Hooks must then be added to external/public loader
   // (so they're triggered in userland)