bpo-43974: Move Py_BUILD_CORE_MODULE into module code (GH-29157)

setup.py no longer defines Py_BUILD_CORE_MODULE. Instead every
module defines the macro before #include "Python.h" unless
Py_BUILD_CORE_BUILTIN is already defined.

Py_BUILD_CORE_BUILTIN is defined for every module that is built by
Modules/Setup.

The PR also simplifies Modules/Setup. Makefile and makesetup
already define Py_BUILD_CORE_BUILTIN and include Modules/internal
for us.

Signed-off-by: Christian Heimes <christian@python.org>