Eliminate `pybindit` namespace (#5607) * Factor out pybind11/detail/pybind11_namespace_macros.h from pybind11/detail/common.h * Eliminate `pybindit` namespace, use pybind11/detail/pybind11_namespace_macros.h instead. * Fix oversight: add pybind11_namespace_macros.h in tests/extra_python_package/test_files.py * [skip ci] Add copyright notice.