hugolib: Add the last lookup variant for the GetPage index

hugolib: Add the last lookup variant for the GetPage index

Now, even for nun-bundles it is possible to do lookup without path or extension.

So, given `blog/my-blog-post.en.md` these lookups will succeed:

* `blog/my-blog-post.en.md`
* `blog/my-blog-post`
* `my-blog-post.en.md`
* `my-blog-post`

See #4312
See https://github.com/gohugoio/hugoDocs/issues/307