bpo-35081: Add pycore_ prefix to internal header files (GH-10263)

* Rename Include/internal/ header files:

  * pyatomic.h -> pycore_atomic.h
  * ceval.h -> pycore_ceval.h
  * condvar.h -> pycore_condvar.h
  * context.h -> pycore_context.h
  * pygetopt.h -> pycore_getopt.h
  * gil.h -> pycore_gil.h
  * hamt.h -> pycore_hamt.h
  * hash.h -> pycore_hash.h
  * mem.h -> pycore_mem.h
  * pystate.h -> pycore_state.h
  * warnings.h -> pycore_warnings.h

* PCbuild project, Makefile.pre.in, Modules/Setup: add the
  Include/internal/ directory to the search paths of header files.
* Update includes. For example, replace #include "internal/mem.h"
  with #include "pycore_mem.h".
diff --git a/Objects/iterobject.c b/Objects/iterobject.c
index 5f5ebfc..23f0639 100644
--- a/Objects/iterobject.c
+++ b/Objects/iterobject.c
@@ -1,8 +1,8 @@
 /* Iterator objects */
 
 #include "Python.h"
-#include "internal/mem.h"
-#include "internal/pystate.h"
+#include "pycore_mem.h"
+#include "pycore_state.h"
 
 typedef struct {
     PyObject_HEAD