What problem does this feature solve?

When using esmExternalRequirePlugin, people might accidentally forget to remove the dependency from the top-level external options, the plugin won't work. Example:

import { defineConfig } from "rolldown";
import { esmExternalRequirePlugin } from "rolldown/experimental";

export default defineConfig({
  input: "src/index.js",
  external: ["react"],
  plugins: [
    esmExternalRequirePlugin({
      external: ["react"],
    }),
  ],
});

Related: #6022 (comment)

What does the proposed API look like?

Just throw an error (or warn) in that case, inform the user about the miscommunication.