Move GoalRunner to the bin package, together with pants_exe.py.

Gets rid of the command package.

Testing Done:
Pants builds. Dev and regular pants work.

Reviewed at https://rbcommons.com/s/twitter/r/1471/
Sync of https://github.com/pantsbuild/pants/commit/f3b15f9073ef86802614ee5659f0f7db9249a187