Fix unpack directory contains symlinks on py3k

| Q             | A
| -------------:| ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Tests pass?   | yes
| Fixed tickets | #611
| License       | MIT

Hello,

On python3 installation fail when the source directory contains symlinks.
See: https://api.travis-ci.org/jobs/4900020/log.txt?deansi=true

Before apply this fix tests fails on py32 and py33. https://travis-ci.org/alquerci/pip/builds/4980091

Symbolic links are not followed.