fixup! fixup! download: clean-up, fix, optimization

Commit c3903c7e was error-prone
because (lambda f1, f2: False) may be wrong in other cases than where
it's currently used.

src/zc/buildout/download.py

@@ -40,10 +40,6 @@ import re
 import shutil
 import tempfile
 import zc.buildout
-try:
-    from os.path import samefile
-except ImportError:
-    samefile = lambda f1, f2: False
 from . import bytes2str, str2bytes
 from .rmtree import rmtree
 
@@ -166,7 +162,8 @@ class Download(object):
                 self.logger.debug("Fallback to cache using %s",
                                   cached_path, exception=1)
             else:
-                if not samefile(path, cached_path):
+                samefile = getattr(os.path, 'samefile', None)
+                if not (samefile and samefile(path, cached_path)):
                     # update cache
                     try:
                         os.remove(cached_path)