Sort out stack derived pools during refresh.
Fix a merge problem.
Add source consumer to pool.

For stack derived pools, the link to a consumer if extremely weak, we
have to assume the requires_host attribute is set and that may not
always be the case for derived pools. Store it explicitly instead for
both stack, and entitlement derived pools.
Rename linked stack to source stack for consistency.

Matched sourceEntitlement today.
Move stack derived code into pool rules.

This is a good fit for refresh pools code, and has some potential for
overlap with normal update pools path which I'm still trying to get
sorted out.