Add stage-packages to current part
Add stage-packages to current part

In all examples, we have made the stage-packages runtime as part of the
corresponding snapcraft part using it. Let's keep the same semantic.

Winael/snappy-playpen-1,elopio/snappy-playpen,Zap123/snappy-playpen,elopio/snappy-playpen,wandrewkeech/snappy-playpen,dplanella/snappy-playpen,jamestait/snappy-playpen,dplanella/snappy-playpen,wandrewkeech/snappy-playpen,jamestait/snappy-playpen,ubuntu/snappy-playpen,ubuntu/snappy-playpen,ubuntu/snappy-playpen,jamestait/snappy-playpen,Zap123/snappy-playpen,elopio/snappy-playpen,elopio/snappy-playpen,Winael/snappy-playpen-1,jamestait/snappy-playpen,ubuntu/snappy-playpen