This is now the preferred way of providing plugin metadata. It's not just a cosmetic change because there is an actual problem with the original: the desktop file kded_touchpad.desktop sets X-KDE-DBus-ModuleName=touchpad which is presumably intended to register the KDED module under that name on DBus; all of the clients access it with the interface path "/modules/touchpad". However, this key appears to be being ignored, and the module is registered under the path "/modules/kded_touchpad" which can be confirmed with qdbusviewer. This incorrect path means that touchpad control via the Plasma applet does not work, and 'kcmshell5 kcmkded' does not show the touchpad module status correctly.
This change updates the KDED plugin to use JSON metadata.