Project Import: `:container_expiration_policy` is not restored correctly

While adding specs for the Import, I found that quite fresh relation, :container_expiration_policy is not being Imported.

The reason is: in our current RelationFactory implementation, for this relation to be restored, it should be included into EXISTING_OBJECT_CHECK and UNIQUE_RELATIONS.

This is very implicit, of course, completely undiscoverable for the person who adds a relation from the product side.

100% coverage and forcing to add specs on any new relation would help, but it would still be very painful for someone not familiar with Import and RelationFactory to discover that, so my proposal is to drop that implicitness and calculate these lists automagically (via reflection and similar).

The first attached MR will just fix and issue and a regression test, but we should eventually drop the implicitness of these "corner cases" and named relations described in the import_export code.