utils/scanpypi: add setup.py script directory as sys.path[0]

utils/scanpypi: add setup.py script directory as sys.path[0]

Even though the directory containing a package's setup.py was added to
sys.path, some setup.py implementations rely on the fact that it is placed
in sys.path[0].

An example package is 'cram' which failed to be added with scanpypi:

    Traceback (most recent call last):
      File "utils/scanpypi", line 756, in <module>
        main()
      File "utils/scanpypi", line 703, in main
        package.load_setup()
      File "utils/scanpypi", line 303, in load_setup
        setup = imp.load_module('setup', s_file, s_path, s_desc)
      File "/usr/lib/python3.8/imp.py", line 234, in load_module
        return load_source(name, filename, file)
      File "/usr/lib/python3.8/imp.py", line 171, in load_source
        module = _load(spec)
      File "<frozen importlib._bootstrap>", line 702, in _load
      File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
      File "<frozen importlib._bootstrap_external>", line 783, in exec_module
      File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
      File "/tmp/scanpypi-2pzc5wb_/python-cram/cram-0.7/setup.py", line 44, in <module>
        long_description=long_description(),
      File "/tmp/scanpypi-2pzc5wb_/python-cram/cram-0.7/setup.py", line 20, in long_description
        return open(os.path.join(sys.path[0], 'README.rst')).read()
    FileNotFoundError: [Errno 2] No such file or directory: '.../buildroot/utils/README.rst'

The corresponding code from cram's setup.py is:

    def long_description():
        """Get the long description from the README"""
        return open(os.path.join(sys.path[0], 'README.rst')).read()

Indeed, the Python documentation says:

https://docs.python.org/3.8/library/sys.html#sys.path
    "...
    As initialized upon program startup, the first item of this list,
    path[0], is the directory containing the script that was used to invoke
    the Python interpreter.
    ..."

Fix this by inserting explicitly at index 0 instead of appending to
sys.path.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>