Convert calibre modules to ebook_converter.

Here is the first batch of modules, which are needed for converting
several formats to LRF. Some of the logic has been change, more cleanups
will follow.

ebook_converter/polyglot/html_entities.py

@@ -2,7 +2,7 @@
 # vim:fileencoding=utf-8
 # License: GPL v3 Copyright: 2019, Eli Schwartz <eschwartz@archlinux.org>
 
-from polyglot.builtins import is_py3
+from ebook_converter.polyglot.builtins import is_py3
 
 if is_py3:
     from html.entities import name2codepoint