Get the folder from the absolute path to the instance

[miguelsousa]

Relates to adobe-type-tools/afdko#753

Traceback happens when filename is just the name of the UFO file with no leading folder path, e.g. <instance familyname="Test" filename="Test-Result.ufo" stylename="Result">

Traceback (most recent call last):
  File "/Users/msousa/Git/UFO/ufoProcessor/Lib/ufoProcessor/__init__.py", line 117, in build
    r = document.generateUFO(processRules=processRules)
  File "/Users/msousa/Git/UFO/ufoProcessor/Lib/ufoProcessor/__init__.py", line 305, in generateUFO
    os.makedirs(folder)
  File "/usr/local/bin/../Cellar/python/3.7.1/bin/../Frameworks/Python.framework/Versions/3.7/lib/python3.7/os.py", line 221, in makedirs
    mkdir(name, mode)
FileNotFoundError: [Errno 2] No such file or directory: ''