force systemsettings for kcms

Summary:
as we can't kill completely kcmshell just yet, hack intothe services runner to
replace on the fly the exec line of the service to systemsettings

BUG: 402790
FIXED-IN: 5.19.0

Test Plan:
search and launch directly for a module, systemsettings is launched wit hthe
proper module loaded

Reviewers: Plasma, ngraham

Reviewed By: ngraham

Subscribers: broulik, ngraham, davidre, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D29157