Dropping the schema in which Citus has been created fails if the schema was marked as distributed

Documenting a probably known/old issue:

```sql
SELECT citus_add_node('localhost', 9701);

-- create a Citus table in public schema to mark public as a distributed object
create table l1 (a int unique);
SELECT create_reference_table('l1');

drop schema public cascade;
NOTICE: drop cascades to 2 other objects
DETAIL: drop cascades to extension citus
drop cascades to table l1
ERROR: cache lookup failed for pg_dist_local_group, called too early?
```

Start by running the SQL sequence in the report, including creating the reference table and dropping the public schema. Trace the Citus extension's schema-drop handling when the schema is marked distributed; done means the schema can be dropped without the pg_dist_local_group cache lookup error.

Written by the indexing model from the issue text.