Replace those calls to `BKE_fcurve_find()` that are searching in the
curves of an Action with their corresponding call in `animrig`:
- `animrig::fcurve_find_in_action()` when it should really search
through the entire Action,
- `animrig::fcurve_find_in_action_slot()` when only the F-Curves for a
specific slot should be searched, or
- `animrig::fcurve_find_in_assigned_slot()` same as above, searching
through the action slot that is assigned to the given ADT.
This also makes `animrig::fcurve_find_in_action()` compatible with both
layered and legacy Actions.
4a1bf5f172