Chop up the hairball a bit more: move some JVM specific tasks into pants/jvm/

The following classes now live in pants.jvm:

* IvyTaskMixin, for Tasks that call ivy_resolve
* JvmToolTaskMixin, for Tasks that bootstrap JVM tools
* NailgunTask
* IvyResolve
* JvmToolBootstrapper

Removing Task.ivy_resolve and the methods provided by JvmToolTaskMixin
cuts a large thread in the hairball--Task is no longer explicitly
dependent upon JVM specific machinery.

In a future update, I'll move the other explicitly JVM Task subclasses from pants.tasks to pants.jvm, or maybe pants.jvm.tasks.

Testing Done:
ci.sh passes

Reviewed at https://rbcommons.com/s/twitter/r/363/