--setup_file needs fuller-path

[kennknowles]

Running --setup_file setup.py fails:

  File "/Users/lambert/Projects/dancedeets-monorepo/dataflow/lib/apache_beam/runners/dataflow/internal/dependency.py",
line 402, in _build_setup_package
    os.chdir(os.path.dirname(setup_file))
OSError: [Errno 2] No such
file or directory: ''

Mainly, because it's attempting to do os.chdir('').

Running --setup_file ./setup.py works fine.

Imported from Jira BEAM-1813. Original Jira may contain additional context.
Reported by: mlambert.