Change typecheck to perform within the current namespace/schema (#1389)

This PR fixes #1388 by doing a test cast for the type and if it fails adding the type.

layers/poi/public_transport_stop_type.sql

@@ -1,12 +1,11 @@
 DO
 $$
     BEGIN
-        IF NOT EXISTS(SELECT 1
-                      FROM pg_type
-                      WHERE typname = 'public_transport_stop_type') THEN
+        PERFORM 'public_transport_stop_type'::regtype;
+    EXCEPTION
+        WHEN undefined_object THEN
             CREATE TYPE public_transport_stop_type AS enum (
                 'subway', 'tram_stop', 'bus_station', 'bus_stop'
                 );
-        END IF;
     END
 $$;