dune_python_install_package: avoid error when pip is not installed, but nothing to install
dune_python_install_package currently checks that
pip was found at the beginning of the function and errors out when it was not found. However, when
DUNE_PYTHON_VIRTUALENV_SETUP is false,
pip would never be called.
It would be nice if
dune_python_install_package would not return an error in this case.