Keep symlinks as symlinks when installing

Without this, "pip install" crashes when encountering symlinks that
point to external paths. The culprit is "shutil.copytree()", which
throws an error in such cases if the "symlinks" parameter is not set to
"True".

This fixes #1006.

Author: myint

pip/download.py

@@ -359,7 +359,7 @@ def unpack_file_url(link, location):
         # delete the location since shutil will create it again :(
         if os.path.isdir(location):
             rmtree(location)
-        shutil.copytree(source, location)
+        shutil.copytree(source, location, symlinks=True)
     else:
         unpack_file(source, location, content_type, link)