Plasma::PluginLoader contains methods for listing/loading of various Plasma related plugins (Applets, DataEngines, ContainmentActions etc).
A lot of this is based on KPluginInfo/KPluginTrader, this should be ported to KPluginMetaData/KPluginLoader.
It defines virtual methods that allow writing custom loaders. This seems to be mostly unused (only ShellPluginLoader (deprecated) and SimpleLoader (for tests)) extend it in KDE code. Might be a good idea to get rid of that then.