fixup! download: clean-up, fix, optimization

To be squashed with commit 4ccb79e9.

src/zc/buildout/download.py

@@ -40,7 +40,12 @@ 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
 
 class netrc(netrc.netrc):
 
@@ -161,7 +166,15 @@ class Download(object):
                 self.logger.debug("Fallback to cache using %s",
                                   cached_path, exception=1)
             else:
-                locate_at(path, cached_path) # update cache
+                if not samefile(path, cached_path):
+                    # update cache
+                    try:
+                        os.remove(cached_path)
+                    except OSError as e:
+                        if e.errno != errno.EISDIR:
+                            raise
+                        rmtree(cached_path)
+                    locate_at(path, cached_path)
                 return path, is_temp
         else:
             self.logger.debug('Cache miss; will cache %s as %s' %