Extend the gaudirun.py GaudiConfig2 file lookup to allow full path to modules

Using the path to a file instead of the module name would be very useful for testing, but it doesn't work:

> gaudirun.py ./test_config.py:config
# setting LC_ALL to "C"
Traceback (most recent call last):
  File "/pool/marcocle/cmake-modernization/Gaudi/Gaudi/scripts/gaudirun.py", line 547, in <module>
    exec(o, g, l)
  File "<string>", line 1, in <module>
  File "/pool/marcocle/cmake-modernization/Gaudi/GaudiKernel/python/GaudiKernel/ProcessJobOptions.py", line 505, in importOptions
    raise ParserError("Unknown file type '%s' ('%s')" % (ext, optsfile))
GaudiKernel.ProcessJobOptions.ParserError: Unknown file type '.py:config' ('./test_config.py:config')