Migrate Publisher.isBlocked uses before removing the field.

[isoos]

• Fix integrity check wrt. blocked packages #8336
• The PublisherBackend.getPublisher relied on isBlocked, but not the isModerated flag. Its uses needed a review whether their method needs the Publisher entity only when it is visible or regardless of its visibility state. The first commit contains only that update.
• The other two commits are more straightforward removal of its use from lib and test, and also from migration (+ its test).
• The removal of the field still needs another site release before we can do it.