turbopack: Fix pageExtensions with dots (#58504)
### What?
Fixes finding Page/App pages when using a `pageExtension` that itself contains a `.`. Eg, `pages/index.page.js` should match as a Page when using `pageExtensions: ["page.js"]`
### Why?
Matching webpack configurations.
### How?
We split in reverse direction, meaning we'd have a `(basename, ext)` of `('index.page', 'js')`. Webpack [uses forward splitting](https://github.com/vercel/next.js/blob/9ab8828f72e96f5de86a7c50b67a1c5abe6e146c/packages/next/src/build/webpack/loaders/next-metadata-route-loader.ts#L27-L31), giving a proper `('index', 'page.js')`.
Closes PACK-1970
Fixes https://github.com/vercel/turbo/issues/6106
Fixes https://github.com/vercel/next.js/issues/57603
