[Feature]: Support same filename "index.css" "index.js" in`"bundle": false`

[SoonIter]

What problem does this feature solve?

import { pluginReact } from '@rsbuild/plugin-react';
import { type LibConfig, defineConfig } from '@rslib/core';

export default defineConfig({
  source: {
    entry: {
      index: ['./src/**', '!./src/env.d.ts'],
    },
  },
  lib: [
    {
      format: 'esm',
      bundle: false,
    },
  ],
  plugins: [pluginReact()],
});

What does the proposed API look like?

fix this