fix kpluginindex caching

This commit fixes creating the plugin index that is used -- when
available - for reading plugins. In detail:

• Index plasma subdirectory when no packageroot is specified
• Iterate of its subdirectories instead of just picking the highest

level (packages are not installed there)

• Add an option to remove the caches
• some cleanups

Reviewed-by: Marco Martin <notmart@gmail.com>