fix(rollup): de-default default export from stub

[danielroe]

Testing latest unbuild with module-builder:

const _module = await jiti.import("/Users/daniel/code/nuxt/fonts/src/module.ts");

export default _module;

... and then

nuxtModule = await jiti.import(src, { default: true });

nuxtModule is '[object AsyncFunction]' and you access the module from nuxtModule.default

[pi0]

LGTM. We might also fix more root causes in jiti but this seems valid for stub..