bpo-38353: Cleanup includes in the internal C API (GH-16548)

Use forward declaration of types to avoid includes in the internal C
API. Add also comment to justify other includes.
diff --git a/Modules/main.c b/Modules/main.c
index 929417f..0288f17 100644
--- a/Modules/main.c
+++ b/Modules/main.c
@@ -2,6 +2,7 @@
 
 #include "Python.h"
 #include "pycore_initconfig.h"
+#include "pycore_pathconfig.h"
 #include "pycore_pylifecycle.h"
 #include "pycore_pymem.h"
 #include "pycore_pystate.h"