gh-106320: Remove private pythonrun API (#108599)

Remove these private functions from the public C API:

* _PyRun_AnyFileObject()
* _PyRun_InteractiveLoopObject()
* _PyRun_SimpleFileObject()
* _Py_SourceAsString()

Move them to the internal C API: add a new pycore_pythonrun.h header
file. No longer export these functions.

Python/bltinmodule.c

@@ -12,6 +12,7 @@
 #include "pycore_object.h"        // _Py_AddToAllObjects()
 #include "pycore_pyerrors.h"      // _PyErr_NoMemory()
 #include "pycore_pystate.h"       // _PyThreadState_GET()
+#include "pycore_pythonrun.h"     // _Py_SourceAsString()
 #include "pycore_sysmodule.h"     // _PySys_GetAttr()
 #include "pycore_tuple.h"         // _PyTuple_FromArray()
 

Python/pythonrun.c

@@ -21,6 +21,7 @@
 #include "pycore_pyerrors.h"      // _PyErr_GetRaisedException, _Py_Offer_Suggestions
 #include "pycore_pylifecycle.h"   // _Py_UnhandledKeyboardInterrupt
 #include "pycore_pystate.h"       // _PyInterpreterState_GET()
+#include "pycore_pythonrun.h"     // define _PyRun_InteractiveLoopObject()
 #include "pycore_sysmodule.h"     // _PySys_Audit()
 #include "pycore_traceback.h"     // _PyTraceBack_Print_Indented()