Add missing index renaming when completing a migration (#577)

During testing the faithful duplication of indexed columns I discovered
that
if a column is included in multiple indices the only one of the indices
are renamed. It happened because the index was deleted from the
bookkeeper in the table even if the index was not renamed.

```
postgres=# \d items
                                          Table "public.items"
 Column |          Type          | Collation | Nullable |                    Default
--------+------------------------+-----------+----------+-----------------------------------------------
 id     | integer                |           | not null | nextval('_pgroll_new_items_id_seq'::regclass)
 city   | character varying(255) |           |          |
 name   | character varying(255) |           |          |
Indexes:
    "_pgroll_new_items_pkey" PRIMARY KEY, btree (id)
    "_pgroll_dup_idx_items_unique_name" UNIQUE, btree (name)
    "items_unique_name_id" UNIQUE CONSTRAINT, btree (city, name)

```

Correct output after my fix:

```
postgres=# \d items
                                          Table "public.items"
 Column |          Type          | Collation | Nullable |                    Default
--------+------------------------+-----------+----------+-----------------------------------------------
 id     | integer                |           | not null | nextval('_pgroll_new_items_id_seq'::regclass)
 city   | character varying(255) |           |          |
 name   | character varying(255) |           |          |
Indexes:
    "_pgroll_new_items_pkey" PRIMARY KEY, btree (id)
    "idx_items_unique_name" UNIQUE, btree (name)
    "items_unique_name_id" UNIQUE CONSTRAINT, btree (city, name)

```

pkg/migrations/rename.go

@@ -138,6 +138,9 @@ func RenameDuplicatedColumn(ctx context.Context, conn db.DB, table *schema.Table
 			if err != nil {
 				return fmt.Errorf("failed to rename index %q: %w", idx.Name, err)
 			}
+
+			// Index no longer exists, remove it from the table
+			delete(table.Indexes, idx.Name)
 		}
 
 		if _, ok := table.UniqueConstraints[StripDuplicationPrefix(idx.Name)]; idx.Unique && ok {
@@ -154,8 +157,6 @@ func RenameDuplicatedColumn(ctx context.Context, conn db.DB, table *schema.Table
 			}
 		}
 
-		// Index no longer exists, remove it from the table
-		delete(table.Indexes, idx.Name)
 	}
 
 	return nil