Fix query for available modules

Summary:
The old query was bad because two reasons:
 * it didn't use the same query systemsettings uses
 * it didn't use exist so if the first property did not exist the second one was not evaluated since the parser bailed out

Test Plan: Ran kcmshell5 --list, it's better now

Subscribers: plasma-devel

Tags: #plasma

BUGS: 378548

Differential Revision: https://phabricator.kde.org/D5355