snap.yaml skill->interface change

[Mark Shuttleworth]

It's not a small change in direction, it's 180 degrees :)

I wonder if this is because some people think of the wall socket as the
plug, and others think of the gadget-prong-thing as the plug. The latter
are technically correct, I think, but using semantics that are not
consistently understood doesn't help us grow consistent understanding.
When we chose "plug" it's because we thought it would help people "get
it", but it isn't doing that, and reversing the directionality won't help.

I wonder if we should instead go to something else we explored, which is
"tangs and slots". If you don't say "plug" it becomes much clearer that
the slot is the receiver, and tang is a nice word in many ways.

The provider of an interface is a tang, the consumer or receiver is a
slot. Snaps have slots which can or must be filled, and tangs which they
can offer other snaps to fill slots.

Mark