Protect cleanup code of plugin unload from deleting any files other than temp files

[rashmigottipati]

There was a case for a stand-alone plugin where all the files in the current working directory were removed as there are no checks provided in the cleanup code of plugin unload in the plugin_manager. We need checks around the cleanup code in plugin_manager.go so that we prevent deletion of any other files other than the temp files that are created to load plugins.