[conan-centre] forbid CONAN_LIBS and CONAN_PKG

[SSE4]

in the near future (conan 2.0), old CMake generators (cmake, cmake_multi, cmake_paths, etc.) will be removed, as well as conan_basic_setup. targets CONAN_PKG:: won't be provided any longer. also, variables like CONAN_LIBS won't be provided as well. these things were mainly used by test_package, and sometimes in patches to provide conan integration. it's time to start removing them.