CMake: don't create empty dir lib/bornagain

The command cmake .. (with or without BA_PY_PACKAGE_VERSIONS option) creates a directory lib/bornagain. This directory remains empty throughout the build process. It has a disturbing consequence: If we build without Python API (pure cmake .. with no BA_PY_PACKAGE_VERSIONS) we would expect the Python command import bornagain to fail. However, if the directory lib/bornagain is present, then import bornagain is executed silently. This can lead to confusion and to strange error reports like #188 (closed).

The strange behavior of Python's import statement can be demonstrated without building BornAgain:

$ rm -rf *
/G/sw/ba/build$ python3
Python 3.9.10 (main, Jan 16 2022, 17:12:18) 
[GCC 11.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import bornagain
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'bornagain'
>>> quit()
/G/sw/ba/build$ mkdir lib
/G/sw/ba/build$ mkdir lib/bornagain
/G/sw/ba/build$ python3
Python 3.9.10 (main, Jan 16 2022, 17:12:18) 
[GCC 11.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import bornagain
>>> repr(bornagain)
"<module 'bornagain' (namespace)>"
>>> help(bornagain)

The last statement yields

Help on package bornagain:

NAME
    bornagain

PACKAGE CONTENTS


FILE
    (built-in)

We can do nothing about Python's weird corners. But we should prevent lib/bornagain from being created.