Fixes #1415.

There is a discrepancy between how the DBAL represents the introspected schema and how the ORM generates the schema that represents entities:

• The DBAL includes all existing namespaces into the introspected schema, including the default ones like public for PostgreSQL.
• The ORM doesn't include any namespaces into the generated schemas, even if the schema contains tables in those namespaces. This behavior is incorrect, but fixing it isn't trivial (Create the default namespace in the schema orm#11658).

The above discrepancy leads to false differences between the schemas, which we want to compensate here:

• As of DBAL 4.0, the PostgreSQL schema manager sets the introspected schema name to the name of the current schema
• If it's set, Migrations will create the corresponding namespace in the schema generated by the ORM.