KPackage Manager: install, list, remove Plasma packages. More information: https://techbase.kde.org/Development/Tutorials/Plasma5/QML2/GettingStarted#Kpackagetool5.
kpackagetool5 --list-types
kpackagetool5 --type package_type --install path/to/directory
kpackagetool5 --type package_type --upgrade path/to/directory
kpackagetool5 --type Plasma/Applet --list --global
kpackagetool5 --type Plasma/Applet --remove "name"