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.
8 files changed