Currently, #include "pymem.h" may include Include/pymem.h or Include/internal/pymem.h depending where is the C file (.c) and if Include/internal/ is in the header search path or not.

I propose to:

* Rename Include/internal/ to Include/pycore/
* In this subdirectory, rename xxx.h to pycore_xxx.h to avoid any risk of confusion
* Automatically include pycore_xxx.h in xxx.h if Py_BUILD_CORE is defined: this should avoid the need of explicit #include "internal/xxx.h" in C files

For example, Include/internal/pystate.h becomes Include/pycore/pycore_pystate.h.

Attached PR implements this idea.

I chose to rename "internal" subdirectory to "pycore" to prepare the addition of other subdirectories. See:

* https://pythoncapi.readthedocs.io/split_include.html
* https://pythoncapi.readthedocs.io/

Next steps:

* Move all code surrounded by #ifdef Py_BUILD_CORE from Include/*.h into Include/pycore/*.h: see the second commit of my PR for an example
* Move core surrounded by #ifndef Py_LIMITED_API from Include/*.h into Include/limited/ (example of filename: Include/limited/limited_object.h)


This change should be backward compatible since Include/internal/ must not be used outside CPython core. If someone does that, well, be ready for breakage :-) It's not supported.