fix(postgres): Primary Keys table helper

[wburklund]

The current Primary Keys table helper for Postgres doesn't list primary keys as expected because it joins against information_schema.referential_constraints, which only has foreign keys.

To reproduce:

1. Spin up the current stable version of PostgreSQL (16.3).
2. Create a table with a primary key:

CREATE TABLE pk_test (
  id INT,
  name TEXT,
  PRIMARY KEY (id)
);

3. Run the existing Primary Keys table helper and see that the primary key is not listed.

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This PR fixes the Primary Keys table helper by simply querying against information_schema.table_constraints. It also renames basic_constraint_query to indicate it should only be used for foreign key queries.