behavior of file path definition on file inexistance rather than directory existance

For now when calling `foo/bar` the system use immediately `foo/bar/index.md` if `foo/bar/` directory exists.
Make more sense to do that instead if the file `foo/bar/index.md` exists.

This allow to have a directory with the same name next to a page file, adding a choice regarding files organization :

    content/
        bar/
            index.md
        foo/
        foo.md

May be usefull for instance in [Pages Images plugin](https://github.com/nliautaud/pico-pages-images#files-location).