ProviderConfig gets deleted while PCUs are still terminating

### What happened?

On an e2e test, I deleted a set of `postgresql.sql.m` resources (allegedly any other flavor applies too), together with its `providerconfig.postgresql.sql.m`.
The `in-use.crossplane.io` finalizer is set whenever the `.status.users` of that provider config (which counts the number of `providerconfigusages.postgresql.sql.` resources) is >0 **should** prevent the provider config from being deleted.

However, we're setting it to 0 even when some resources are still on `Deleting` state. The sequence I see is:

- All resources in terminating state.
- PCU goes down to 0 at some point, even though some resources are `Deleting`.
- ProviderConfig gets deleted (0 users).
- Next reconcile loop: PCU gets recreated (resources are still there) but ProviderConfig is **gone**.

### How can we reproduce it?

Create a composition which bundles some resources + its providerConfig, and do a foreground delete of it.

### What environment did it happen in?
Crossplane version: v2.3
Provider version: v0.15, v0.16beta