Move the runnable state calculation out of the collect_nodes function.
They are really two different steps that doin't overlap much.
The runnable state of a node is very easy to calculate. A node is
runnable if it is linked to another node without a passive port. When we
find two runnable nodes, make them runnable, which makes all nodes
linked to them runnable, stopping at passive ports.
We don't have to check the active state of the nodes or links to group
them together. This ensures we don't swap nodes around too much when the
node or link state changes.
There is no reason to delay preparing the link (by the scheduler) when
both nodes are active, we can do that right from the start.
This makes things a bit more symetrical because deactivating a node does
not unprepare a link.
This however changes things a bit because you can no longer delay link
prepare until you activate the node. I don't know if this is actually in
use and it would probably be to delay format negotiation. The right way
do delay format negotiation is to wait until an EnumFormat is set but
that is something to improve later.
