cmake: move script mode handling from package helper to extensions.cmake

Move Zephyr CMake script mode handling from package_helper.cmake into
extensions.cmake.

This ensures that all Zephyr CMake script which includes
extensions.cmake will have the same functions stubbed or mocked and thus
does not need to replicate this behavior.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>