bpo-40602: Rename hashtable.h to pycore_hashtable.h (GH-20044)

* Move Modules/hashtable.h to Include/internal/pycore_hashtable.h
* Move Modules/hashtable.c to Python/hashtable.c
* Python is now linked to hashtable.c. _tracemalloc is no longer
  linked to hashtable.c. Previously, marshal.c got hashtable.c via
  _tracemalloc.c which is built as a builtin module.
diff --git a/Python/marshal.c b/Python/marshal.c
index b4429ae..d2bff52 100644
--- a/Python/marshal.c
+++ b/Python/marshal.c
@@ -12,7 +12,7 @@
 #include "longintrepr.h"
 #include "code.h"
 #include "marshal.h"
-#include "../Modules/hashtable.h"
+#include "pycore_hashtable.h"
 
 /*[clinic input]
 module marshal