Turbopack: layout segment optimization for Pages (#74815)
Closes PACK-3715
This also required some changes to the dev loader, as since
https://github.com/vercel/next.js/commit/bc05ce9, app and document are
used from the page bundle itself, and not from the app and document.
~~`_app.js` and `_document.js` are chunked first before the page (this
result is then cached and reused across pages).~~
~~Here, `_document.js` which includes lodash is only chunked once as
part of `page3.js` and the other two reuse that.~~
