Postgres imports: Hide postgis internal views

[craigds]

Description

Hides these views/tables from kart import '<postgres-url>':

 extname | extschema |      relname      | relkind
---------+-----------+-------------------+---------
 postgis | public    | spatial_ref_sys   | r
 postgis | public    | geography_columns | v
 postgis | public    | geometry_columns  | v
 postgis | public    | raster_columns    | v
 postgis | public    | raster_overviews  | v

These aren't importable things and they just cause potential problems,
especially when used with --all-tables (but they are noisy when
listing/prompting for tables also)

Related links:

re #434

Checklist:

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