`UNIQUE` constraints are dropped by migrations in non-`public` schemas

[Andrew Farries]

When duplicating columns for backfilling, UNIQUE constraints are dropped when the migration is applied in a schema other than public.

To reproduce:

Run the series of migrations in this gist in a schema other than public, eg:

go run . start --complete --schema foo 01_create_table.json
go run . start --complete --schema foo 02_add_column.json
go run . start --complete --schema foo 03_drop_not_null.json
go run . start --complete --schema foo 04_set_not_null.json

The final schema lacks the unique constraint on the two column:

+----------------+--------------------------+-------------------------+----------+--------------+-------------+
| Column         | Type                     | Modifiers               | Storage  | Stats target | Description |
|----------------+--------------------------+-------------------------+----------+--------------+-------------|
| xata_id        | text                     |  not null               | extended | <null>       | <null>      |
| xata_version   | integer                  |  not null default 0     | plain    | <null>       | <null>      |
| xata_createdat | timestamp with time zone |  not null default now() | plain    | <null>       | <null>      |
| xata_updatedat | timestamp with time zone |  not null default now() | plain    | <null>       | <null>      |
| two            | integer                  |  not null               | plain    | <null>       | <null>      |
+----------------+--------------------------+-------------------------+----------+--------------+-------------+
Indexes:
    "_pgroll_new_tester_pkey" PRIMARY KEY, btree (xata_id)

Preservation of UNIQUE constraints was implemented as part of #227, but missed this specific case of UNIQUE constraints in migrations in non-public schema.