Fix suffix() and unsuffix() to use python builtins (#4912)

* Fix suffix() and unsuffix() to use python builtins

I ran into a strange behaviour with the existing custom
implementation:
 unsuffix("foo.txt") -> "foo"
 unsuffix("foo") -> ""

The python builtins to a seemingly more reasonable thing:
 unsuffix("foo.txt") -> "foo"
 unsuffix("foo") -> "foo"

* Add myself to AUTHORS

Author: sbc100

AUTHORS

@@ -273,4 +273,5 @@ a license to everyone to use it as detailed in LICENSE.)
 * Murphy McCauley <[email protected]>
 * Anatoly Trosinenko <[email protected]>
 * Brad Grantham <[email protected]>
+* Sam Clegg <[email protected]> (copyright owned by Google, Inc.)
 

tools/shared.py

@@ -2336,14 +2336,11 @@ def check_call(cmd, *args, **kw):
     raise
 
 def suffix(name):
-  parts = name.split('.')
-  if len(parts) > 1:
-    return parts[-1]
-  else:
-    return None
+  """Return the file extension *not* including the '.'."""
+  return os.path.splitext(name)[1][1:]
 
 def unsuffixed(name):
-  return '.'.join(name.split('.')[:-1])
+  return os.path.splitext(name)[0]
 
 def unsuffixed_basename(name):
   return os.path.basename(unsuffixed(name))