fix python exports

[nim65s]

Hi,

In #525, we decided to install python.cmake and call FINDPYTHON in generated ${PROJECT_NAME}Config.cmake.

But in those generated files, the include(python.cmake) is after FINDPYTHON(), so it only works for projects that already have it imported it with include(cmake/python.cmake) .

ref. https://github.com/stack-of-tasks/eigenpy/pull/298 for failing test + fix with this PR.

while here:

• also export PYTHON_COMPONENTS, otherwise the FINDPYTHON() will fail in many cases.
• remove the exports of standalone variables, are they are now defined by our FINDPYTHON() call