Move HeadAction out of play.api.controllers

The new `HeadAction` is in a new `play.api.controllers` package.  The problem with this, is it cause the following code, that did compile in Play 2.2.x, to not compile:

```scala
import play.api._
import controllers.foo.Bar
```

`HeadAction` is actually an internal API anyway, so it should be moved to something like `play.core.actions`.