doc: xmlschema extension fixes

* Use xpath() instead of find()/findall(), which do not support
  namespace mappings in older versions of lxml.
* Properly initialize entities dict
doc: Fallback to old behavior for intersphinx

If the object inventories don't exist on disk, fallback to the previous
method of attempting to download them from the specified URL.

Signed-off-by: Sol Jerome <sol.jerome@gmail.com>