general: importing modules from packages fix
general: importing modules from packages fix

* Uses Flask-Registry for importing modules from packages.

Signed-off-by: Eirini Psallida <d8029defc21e6ed23f32bbbd940d30a709d8ff14@cern.ch>
Reviewed-by: Jiri Kuncar <2bdffcb416fed3002465740d19a053a39918cbe5@cern.ch>

inveniosoftware/invenio-base,tiborsimko/invenio-base,inveniosoftware/invenio-base,tiborsimko/invenio-base