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