Was coupled to OGS_USE_PYTHON before.

Also update pip and setuptools on venv creation.

Use just a single find_package()-call.

Renamed OGS_USE_POETRY to OGS_USE_PIP.

No additional tools required, just a Python interpreter.

Only call `poetry add` when there are new Python package requirements.

https://github.com/conan-io/conan/pull/8501

Need to set OGS_VERSION when git is not available.

Packages to install are written to build/.python_packages during CMake
run and then installed once at the end of the CMake run via poetry add.

Speeds up CMake configure time a bit.

Does not interfere with CPM modules.

We currently stick to Snakemake.

Should fix Linux CI runs. asn1crypto is a dependency of cryptography.