Expose `require`d controllers to a directive's controller

[abyx]

Currently you use controllers on directives to expose functionality. Then other directives use those controllers by supplying require and getting the controllers in the link function.

This feels asymmetric and a more natural way would be to allow the injection of required controllers to the directive's controller as well. That would mean that for a lot of use cases would could simply write directives with just the controller function and no need to pass it awkwardly from link to controller.

I was thinking about something like providing a $requires injectable for directive controllers that is basically an object, like $routeParams, from controller name to controller object.

If that sounds reasonable I'll work on a pull request.

[gkalpak]

Duplicate of #5893 and #12989.
Closing, since #5893 has more context/discussion.

_{(There is also PR #6040 addressing this, but it's too old now.)}