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".

Include/internal/pycore_gil.h

@@ -0,0 +1,46 @@
+#ifndef Py_INTERNAL_GIL_H
+#define Py_INTERNAL_GIL_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include "pycore_condvar.h"
+#include "pycore_atomic.h"
+
+#ifndef Py_HAVE_CONDVAR
+#  error You need either a POSIX-compatible or a Windows system!
+#endif
+
+/* Enable if you want to force the switching of threads at least
+   every `interval`. */
+#undef FORCE_SWITCHING
+#define FORCE_SWITCHING
+
+struct _gil_runtime_state {
+    /* microseconds (the Python API uses seconds, though) */
+    unsigned long interval;
+    /* Last PyThreadState holding / having held the GIL. This helps us
+       know whether anyone else was scheduled after we dropped the GIL. */
+    _Py_atomic_address last_holder;
+    /* Whether the GIL is already taken (-1 if uninitialized). This is
+       atomic because it can be read without any lock taken in ceval.c. */
+    _Py_atomic_int locked;
+    /* Number of GIL switches since the beginning. */
+    unsigned long switch_number;
+    /* This condition variable allows one or several threads to wait
+       until the GIL is released. In addition, the mutex also protects
+       the above variables. */
+    PyCOND_T cond;
+    PyMUTEX_T mutex;
+#ifdef FORCE_SWITCHING
+    /* This condition variable helps the GIL-releasing thread wait for
+       a GIL-awaiting thread to be scheduled and take the GIL. */
+    PyCOND_T switch_cond;
+    PyMUTEX_T switch_mutex;
+#endif
+};
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_INTERNAL_GIL_H */