Extract _load_backends

--HG--
extra : amend_source : f3fe9702969d27cefc6c43b33456ef46ca6e8db2
Extract _load_backend

--HG--
extra : amend_source : d6a8e4f3c3cf434d6c86ea31f4720645f90b7ea9
Restore support for demand-import environments
Prefer importlib if available
Fix compatibility importing (now conflicts with 'mod' local)