Fixed 'pants goal targets'

As Jin pointed out in issue 333, pants goal targets is broken. Its pretty easy to patch it up, but it relies on a fixed list of alias-> target classes, which is ugly and seemingly redundant.

Larry says we could just remove it. Or we could try to keep it going. I'm fine with either decision, I just don't like broken commands sitting around, I think it sets bad precedent.

Testing Done:
added test to ci.sh, its green: https://travis-ci.org/pantsbuild/pants/builds/32858261

Bugs closed: 493

Reviewed at https://rbcommons.com/s/twitter/r/914/