Port away from deprecated KPluginLoader methods
Fixes a bunch of warnings about deprecated methods when building with fresh KF5, like:
src/modulesmodel.cpp:97:47: warning: ‘static QString KPluginLoader::findPlugin(const QString&)’
is deprecated: Since 5.86. Contruct a QPluginLoader and call fileName() or use
KPluginMetaData::findPluginById instead [-Wdeprecated-declarations]
Edited by Alexey Min