Add BUILD files to pants

This adds BUILD files to pants.  I've added what I feel to be the minimum
number of additional changes to give BUILD sanity (this means doing imports
directly from modules rather than parent modules in virtually all cases.)

There are a lot of # XXX(fixme) that should be cleaned up in subsequent
reviews.  There's some clear spaghetti that should be fixed (for example,
cross / quasi circular dependencies between base, reporting, goal, task,
etc.)  Adding BUILDs has just made that more obvious than ever.

Testing Done:
PANTS_VERBOSE=1 ./pants py src/python/twitter/pants/bin:pants_old -- tests/python/twitter/pants:all

Reviewed at https://rbcommons.com/s/twitter/r/32/