change root file detection strategy

If no default html file is found in zip (web/index.html, index.html, etc.) there is now a fallback (we need it to be able to integrate HotPotatoes) :

- We scan directories of zip to find HTML files
- If a unique HTML file is found, zip is valid and this file is used as root of the WebResource

NOTE : There is an unmanaged use case : if there is no default HTML file and many HTML files in zip.

In this case, I think we have to return the list of all HTML files found to user and let him choose which is the root file.

I will try to handle this case if I have time and if this feature is usefull.