revdep_rebuild: replace portage.os with stdlib os

portage.os is a re-export of the stdlib os module, kept for Python 2
compatibility. Import os directly.

Signed-off-by: Matt Turner <mattst88@gentoo.org>
Part-of: #62

Author: mattst88

pym/gentoolkit/revdep_rebuild/cache.py

@@ -2,7 +2,7 @@
 Functions for reading, saving and verifying the data caches
 """
 
-from portage import os
+import os
 import time
 
 from portage.output import red

pym/gentoolkit/revdep_rebuild/collect.py

@@ -3,7 +3,7 @@
 """Data collection module"""
 
 import re
-from portage import os
+import os
 import glob
 import stat