[WIP] bpo-35081: Include/internal/ becomes Include/pycore/ by vstinner · Pull Request #10143 · python/cpython

* Rename Include/internal/ directory to Include/pycore/
* Rename Include/pycore/xxx.h to Include/pycore/pycore_xxx.h
* Include/internal/mem.h renamed to Include/pycore/pycore_objimpl.h
* "xxx.h" now includes "pycore/pycode_xxx.h" if Py_BUILD_CORE is
  defined: it is no longer needed to include directly "pycore_xxx.h".
  For example, include "pycore/pycore_objimpl.h" from "objimpl.h".
* objimpl.h now includes pycore_objimpl.h just after defining
  PyGC_Head type. Replace Remove _PyGC_generation0: replaced by
  _PyRuntime.gc.generation0.

pystate: move Py_BUILD_CORE code into pycore_pystate.h