Move code out of goal/__init__.py into goal/goal.py.

We want all our __init__.py files to be empty, and this one actually
illustrates why: the target containing __init__.py depends on phase.py,
and has that dep explicitly in the BUILD file. However you can't load
phase.py without executing __init__.py, so there's also an implicit
dep in the other direction that you can't express in the BUILD file
without causing a circular dep.