hugolib: Avoid index.md in /index/index.html

hugolib: Avoid index.md in /index/index.html

Hugo 0.20 broke some sites that grouped their blog post and images together in subfolders.

This commit re-introduces that behaviour:

* If the file base name resolves to the same as the base name for the output type (i.e. "index" for HTML), the user probably meant it, so we treat that as an `uglyURL`, i.e. `my-blog-post-1.md`=> `/my-blog-post-1/index.html`
* The main use case for this is to group blog post and images together.
* Note that for the top level folder there will be a potential name conflict with a `section` `index.html` (if enabled)
* This issue will not be relevant for subfolders in sections
* Hugo will soon add support for nested sections, but we will have to find a way to separate them from the rest (`/content/_mysubsection` maybe).

Fixes #3396