How to get info about relations between system tables?

One my customer asked me "what tables holds references to table pg_roles"?

The reply is simple - none. pg_roles is view. But this view uses very important table pg_authid. For custom tables we can use a query:

SELECT conname, conrelid::pg_catalog.regclass AS ontable,
       pg_catalog.pg_get_constraintdef(oid, true) AS condef
  FROM pg_catalog.pg_constraint c
 WHERE confrelid = 'a'::pg_catalog.regclass
       AND contype = 'f'
ORDER BY conname;
┌─────────────┬─────────┬─────────────────────────────────────┐
│   conname   │ ontable │               condef                │
╞═════════════╪═════════╪═════════════════════════════════════╡
│ b_a_id_fkey │ b       │ FOREIGN KEY (a_id) REFERENCES a(id) │
└─────────────┴─────────┴─────────────────────────────────────┘
(1 row)

But this method doesn't work for system tables. These tables doesn't use explicitly defined foreign keys. We should to use different method. The system function pg_get_catalog_foreign_keys returns all referencies between system tables, and we can filter result:

SELECT * 
 FROM pg_get_catalog_foreign_keys()
WHERE pktable = 'pg_authid'::regclass;

┌─────────────────────────┬──────────────┬───────────┬────────┬──────────┬────────┐
│         fktable         │    fkcols    │  pktable  │ pkcols │ is_array │ is_opt │
╞═════════════════════════╪══════════════╪═══════════╪════════╪══════════╪════════╡
│ pg_proc                 │ {proowner}   │ pg_authid │ {oid}  │ f        │ f      │
│ pg_type                 │ {typowner}   │ pg_authid │ {oid}  │ f        │ f      │
│ pg_class                │ {relowner}   │ pg_authid │ {oid}  │ f        │ f      │
│ pg_operator             │ {oprowner}   │ pg_authid │ {oid}  │ f        │ f      │
│ pg_opfamily             │ {opfowner}   │ pg_authid │ {oid}  │ f        │ f      │
│ pg_opclass              │ {opcowner}   │ pg_authid │ {oid}  │ f        │ f      │
│ pg_language             │ {lanowner}   │ pg_authid │ {oid}  │ f        │ f      │
│ pg_largeobject_metadata │ {lomowner}   │ pg_authid │ {oid}  │ f        │ f      │
│ pg_statistic_ext        │ {stxowner}   │ pg_authid │ {oid}  │ f        │ f      │
│ pg_event_trigger        │ {evtowner}   │ pg_authid │ {oid}  │ f        │ f      │
│ pg_namespace            │ {nspowner}   │ pg_authid │ {oid}  │ f        │ f      │
│ pg_conversion           │ {conowner}   │ pg_authid │ {oid}  │ f        │ f      │
│ pg_database             │ {datdba}     │ pg_authid │ {oid}  │ f        │ f      │
│ pg_db_role_setting      │ {setrole}    │ pg_authid │ {oid}  │ f        │ t      │
│ pg_tablespace           │ {spcowner}   │ pg_authid │ {oid}  │ f        │ f      │
│ pg_auth_members         │ {roleid}     │ pg_authid │ {oid}  │ f        │ f      │
│ pg_auth_members         │ {member}     │ pg_authid │ {oid}  │ f        │ f      │
│ pg_auth_members         │ {grantor}    │ pg_authid │ {oid}  │ f        │ f      │
│ pg_ts_config            │ {cfgowner}   │ pg_authid │ {oid}  │ f        │ f      │
│ pg_ts_dict              │ {dictowner}  │ pg_authid │ {oid}  │ f        │ f      │
│ pg_extension            │ {extowner}   │ pg_authid │ {oid}  │ f        │ f      │
│ pg_foreign_data_wrapper │ {fdwowner}   │ pg_authid │ {oid}  │ f        │ f      │
│ pg_foreign_server       │ {srvowner}   │ pg_authid │ {oid}  │ f        │ f      │
│ pg_user_mapping         │ {umuser}     │ pg_authid │ {oid}  │ f        │ t      │
│ pg_policy               │ {polroles}   │ pg_authid │ {oid}  │ t        │ t      │
│ pg_default_acl          │ {defaclrole} │ pg_authid │ {oid}  │ f        │ f      │
│ pg_collation            │ {collowner}  │ pg_authid │ {oid}  │ f        │ f      │
│ pg_publication          │ {pubowner}   │ pg_authid │ {oid}  │ f        │ f      │
│ pg_subscription         │ {subowner}   │ pg_authid │ {oid}  │ f        │ f      │
│ pg_variable             │ {varowner}   │ pg_authid │ {oid}  │ f        │ f      │
└─────────────────────────┴──────────────┴───────────┴────────┴──────────┴────────┘
(30 rows)