Fix bug that psqldef attempts to drop&add pk every time if two tables have same name and pk

[yuku]

Suppose there are two tables having:

• same table name
• same primary key column name

-- e.g.
create table user.accounts (id int primary key);
create table bank.accounts (id int primary key);

psqldef generates following DDL every time:

-- Apply --
ALTER TABLE "user"."accounts" DROP CONSTRAINT "accounts_pkey";
ALTER TABLE "user"."accounts" ADD primary key ("id");
ALTER TABLE "bank"."accounts" DROP CONSTRAINT "accounts_pkey";
ALTER TABLE "bank"."accounts" ADD primary key ("id");

This PR fixes this issue.

[yuku]

@k0kubun Could you review this PR? 🙏

FYI: I bet there is almost same issue around getForeignDefs but I couldn't write regression tests, so I left it as is.

https://github.com/k0kubun/sqldef/blob/e0b45dd6f40f69d00c119c51c6514e24f8239a11/adapter/postgres/postgres.go#L415-L431

[k0kubun]

Thanks!