Make consistent use of Python targets and variables in CMake

This is a small clean up on python handling on CMake:

• Use Python3::Module instead of raw handling of include directories and flags
  • This already includes the flags needed to make modules work on macOS
• Use the Python3::Python for embedding python into C++
  • Note that the presence of bindings does not imply the presence of Python3::Python
• Use hashes on current directory to make internal targets unique
  • This allows to configure different python projects
• Promote DUNE_PYTHON_pip_FOUND and DUNE_PYTHON_VIRTUALENV_EXECUTABLE into internal cache variables to have global scope.
  • This is to be consistent with include_guard(GLOBAL).