Allow string dependencies, don't require pants('')

Summary:
There's really no need to wrap every dependency in every
BUILD file with a call to pants() to make a Pants target. Add a
utility function "resolve" which converts:

        - string literal to a Pants() target
        - single Pants target is left alone
        - list of strings or Pants targets to a list of Pants targets

This allows editors of BUILD files to optionally omit the "pants()" boilerplate on each dependency.

In a follow-on commit, I remove such calls from all BUILD files in the pants project.