Bypass empty pages folder for layouts (#40132)
Check `pagesDir` to bypass empty pages folder when appDir is enabled
* Output empty loadable manifest for now if there's no `pagesDir`
* No custom aliases with all page extensions for `/_app`, `_document` if pagesDir is empty, only keep the built-in ones
* Check pagesDir in build/dev-server/eslint
* Type safe: change arguments of some APIs from optional to required, so that we won't mess up with default arguments
