This reverts commit 229ae27c.

Adrian Greeve authored Feb 19, 2018 and Jake Dallimore committed Feb 22, 2018

Fixed the writer so that doesn't create an empty
folder if there is no data to export.

Adds another level of interfaces extending data_provider, namely
core_data_provider and shared_data_provider. Makes it easier to locate
components providing data to core, and the subsystem's manager.

Thanks to Gareth J Barnard for this suggestion.

AMOS BEGIN
 CPY [userpreferences,grades],[userpreferences,moodle]
AMOS END