Rename arg_handler.py package.

The previous name (single_project_pull_arg_handler) was both uber
verbose, and focused attention on the wrong thing.  It originally
made sense when I'd conceived of this project as having multiple
scripts, each one wrapping and adding functionality to the last (with
the smallest one being 'single_project_pull').  But since that
architecture never really panned out, the name no longer makes sense.