As part of this task we should:

1. Make index discriminators that we store within the PlanCache as part of PlanCacheIndexabilityState available outside of the PlanCache. That would mean moving PlanCacheIndexabilityState into CollectionQueryInfo.
2. Refactor PlanCache and remove any references to CanonicalQuery.
3. Change the classic PlanCacheKey implementation so that it can be constructed from a CanonicalQuery and a Collection and change the call sites to construct and pass a PlanCacheKey rather than a CanonicalQuery to the PlanCache API.