Rename Phase -> Goal.

This change deals with phase.py (now goal.py) and things that
depend on it.

CI passes: https://travis-ci.org/pantsbuild/pants/builds/32484729

Future mop-up changes will fix extant uses of the term 'phase'
in various files, and also fix documentation appropriately.

Reviewed at https://rbcommons.com/s/twitter/r/887/
Rename the ':phase' target to ':goal'.

Reviewed in https://rbcommons.com/s/twitter/r/890/.