The new code uses the (internal) BGG JSON API directly.
This removes many dependencies, speeds up the process, makes it more
reliable and makes it possible to handle several copies of a game at
the same time.

When "collid" is empty or --force-new is given, the importer creates
new copies of the games. Without --force-new and with non-empty
"collid" matching existing copies of the games will be updated.

There might be other viable alternatives:

  - Create new copies by default, and introduce some --update switch
    to update the existing collection.

  - Add a completely new command for updating the collection, like e.g.
    collection-update.

  - Detect if the "collid" to be imported is already in the collection.
    Then update if it is, or add a copy if it is not. This solution
    probably exhibits the least surprising behavior for the user, but
    it requires an additional request per imported record or a complete
    export beforehand.