Make split-core2core more robust

Created by: dredozubov

I tried using this script to inspect the core in https://ghc.haskell.org/trac/ghc/ticket/14338 and made a few changes fixing this script for me.

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This change is 

split-core2core.py

-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 import sys
 import os
 import re
 
 def split_file(f):
-    base = os.path.splitext(f)[0]
+    base = os.path.basename(f)[0]
     i = 0
 
     print(f)
-    os.mkdir('%s.split' % f)
+    os.makedirs('%s.split' % f)
     output = open('%s.split/%s-00' % (f, base), 'w')
     for ln in open(f):
         if ln.startswith('======='):