Dropping cascading objects may skip invalidating foreign key graph

We've implemented a graph which holds the transitivity of the foreign
key relations with #2211.

The main logic of invalidation is that as soon as a foreign key
created or dropped, invalidate and re-form the graph.

However, we've realized in some cases, Citus skips invalidating the
graph, when any object cascades to dropping a foreign key.

We're expecting some edge cases such as:

- `DROP TYPE` cascades to a column, which cascades to a forein key
- `DROP INDEX` cascades to a forein key

There might be some other edge cases. The propoer solution would be
write a function similar to `findDependentObjects()` which for any
given `DROP` command checks whether any foreign key depends on it.

(Note that the naive solution would be to invalidate the graph on any
`DROP` command. However, we didn't want to go that way since the
invalidation is quite costly.)