Fix esm module import warning for middleware loader (#51014)

We need to spread the module into an object instead of access the module exports that could not exist, to avoid triggering the warning. Fix the bad loader change introduced in #50548 

```
Attempted import error: './middleware.js' does not contain a default export (imported as '
mod').
- info Using locally built binary of @next/swc
- warn You are using an experimental edge runtime, the API might change.
- wait compiling...
- warn ../../../../packages/next/dist/build/webpack/loaders/next-middleware-loader.js?abso
lutePagePath=%2FUsers%2Fhuozhi%2Fworkspace%2Fnext.js%2Ftest%2Fe2e%2Fapp-dir%2Fapp%2Fmiddle
ware.js&page=%2Fmiddleware&rootDir=%2FUsers%2Fhuozhi%2Fworkspace%2Fnext.js%2Ftest%2Fe2e%2F
app-dir%2Fapp&matchers=!
Attempted import error: './middleware.js' does not contain a default export (imported as '
mod').
- wait compiling...
```

packages/next/src/build/webpack/loaders/next-middleware-loader.ts

@@ -42,8 +42,9 @@ export default function middlewareLoader(this: any) {
   return `
         import 'next/dist/esm/server/web/globals'
         import { adapter } from 'next/dist/esm/server/web/adapter'
-        import * as mod from ${stringifiedPagePath}
+        import * as _mod from ${stringifiedPagePath}
 
+        const mod = { ..._mod }
         const handler = mod.middleware || mod.default
 
         if (typeof handler !== 'function') {