Optionally prefer system wide pybind11

[StefanBruens]

Linux distributions typically prefer system provided libraries, so optionally use it when found.

(This also allows to use the github provided tarball, which omits the pybind11 submodule).

Fix the PYBIND_INCLUDE_DIR usage:

• remove misleading find_path invocation, which may point to the system wide pybind11
• use pybind11_INCLUDE_DIR which is provided by both find_package(pybind11) and bundled pybind11/CMakeLists.txt
• pybind11_INCLUDE_DIR is used by pybind11_add_module, use it also for ngcore (core/register_archive.hpp)