[meta] Convert info hooks to plugins

The children all have the appropriate priorities now (thanks @tim.plunkett!) so demoting this one.

No idea why this is or the child issues were critical, please explain the reasoning.

I think the evaluation of priority for each child issue needs to happen on a case by case basis. I agree that 100% conversion of info hooks to plugins isn't a requirement for D8 being functional, so none of the child issues should be major or critical simply due to being a child of this meta. They may deserve that priority on their own merits though.

However, regardless of priority, we also need a decision on whether they deserve an "Approved API change" tag. From http://buytaert.net/drupal-8-api-freeze:

Between API freeze and beta 1, removing temporary backward compatibility layers and deprecated Drupal 7 functions is allowed and encouraged. After beta 1, we get more strict about backward compatibility breaks (temporary backward compatibility layers and deprecated functions not removed by then might not be eligible for removal any more).

I'd argue that info hooks with magic array keys containing function names as values is a deprecated D7 approach, and that plugins are the proper D8 approach, so therefore, those issues deserve to be approved for API change at least until we release beta 1. But I have no authority to actually decide that.

I think this meta issue has lived out its usefulness, so closing. There might be a few info hooks left that haven't been converted, so if someone wants to reopen this meta issue for 8.1.x or 9.x to track those, go for it.