no missing venv workarounds, pip install -r used, libx for manual libs

This commit reduces complexity of the pip install process of
requirements by always depending on virtualenv which allows us
to use pip install -r requirements.txt.
Manually installed libs in main/libx are not touched but have
precedence.
Merge branch 'pip_joern' into pip_lib

Conflicts:
	run.py
Removed double insert libx.