Fix for bug introduced in #3426, which effectively makes start_dir values that were specified for extensions be totally ignored (because the existence check on the start_dir value is typically using a relative value but from the wrong location).

The source tarball for the barbar test easyconfig has been changed to catch this bug: the source file has been moved into a subdirectory src and a README file was added to ensure that the top-level directory is initially picked as start directory after unpacking.

edit: fixes problems like this which occur now when using framework develop when installing an easyconfig file that includes an extension with a custom value for start_dir:

== installing extension protobuf-python 3.6.1 (1/13)...
== FAILED: Installation ended unsuccessfully (build directory: /tmp/easybuild_build/TensorFlow/1.13.1/foss-2018b-Python-3.6.6): build failed (first 300 chars): cmd " /software/Python/3.6.6-foss-2018b/bin/python setup.py build " exited with exit code 2 and output:
/software/Python/3.6.6-foss-2018b/bin/python: can't open file 'setup.py': [Errno 2] No (took 10 sec)

The specified 'start_dir': 'python' is blatantly ignored, and hence setup.py is not found...