KPluginMetaData: Add enum flag for reusing plugins

Loading the plugins & parsing the metadata is expensive compared to listing the files on disk.
In case the findPlugins call is done repeatedly, we often want to ensure that we have found potentially new plugins.
For example, KRunner should pick up newly installed KCMs when querying it for the next time. But KCMs
changing their metadata completely is not any known case.
Other usecases are KParts, KProtocolInfoFactory and potentially potentially Plasma Applets