cubicweb #17076486 Computed relations are broken when setting postgres schema (db-namespace) [open]

When querying a Postgresql db using a given namespace (pg schema), this later do look or tables in the given schema but fall back to default schema (public). Current implementation of deserialize_schema won't mimic this behaviour, preventing from detecting computed relations when using a pg database with a specific pg schema but the cw_cwcomputedrtype table living in the 'public' pg schema.

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