Fix add-dataset for PostGIS working copies

[olsen232]

Unlike for GPKGs, we need to be looking for tables only in the appropriate schema. Happily, the code is all mostly in-place. Added test.

Related links:

Fixes https://github.com/koordinates/kart/issues/965

Checklist:

• [x] Have you reviewed your own change?
• [x] Have you included test(s)?
• [x] Have you updated the changelog?