Some distributions have incompatible type of files with same name
on the root directory. For example, matplotlib-2.0.2.tar.gz has a
directory named "LICENSE" on the root, which may conflict many
other distributions, causing errors like:

  IOError: [Errno 20] Not a directory: u'/tmp/tmpz/LICENSE/LICENSE_STIX'

So we need to isolate unpacking directory of each distributions.

Fixing errors in https://ci.appveyor.com/project/jazzband-bot/pip-tools/build/1.0.174