Allow a task type know its own options scope.

E.g., JavaCompile should know that its options are under compile.java.

Also fix some stale comments from the goal->task terminology change.

Testing Done:
Ran all the tests. Tested that this works when registering options in the new style (will be sent for review in a future change).

Reviewed at https://rbcommons.com/s/twitter/r/1095/
Hook new-style registration up to the goal runner.

Provides appropriate special-casing for GroupTask.

Note: relies on https://rbcommons.com/s/twitter/r/1095/.

Testing Done:
Ran all tests. Manually tested new-style registration on top of this.

Reviewed at https://rbcommons.com/s/twitter/r/1096/
Sync of https://github.com/pantsbuild/pants/commit/e03352dbaf1726232b58e6a47db3d5085ed837f8