Don't support submodule

I have a `node.importmap` as follows:

```
{
  "imports": {
    "react-dom": "https://esm.sh/react-dom@16.13.1"
  }
}
```

Here is a file to test:

```
import ReactDOM from 'react-dom'
import ReactDOMServer from 'react-dom/server.js'
```

The second line can't work.