rez-pip NO packages were installed

Hi,

when installing Qt.py package with rez-pip, it fails.
```
C:\Users\daniel.asztalos>rez-pip --install Qt.py
15:57:58 INFO Using pip-1.5.6 (C:\Users\daniel.asztalos\packages\pip\1.5.6\package.py[0])
Downloading/unpacking Qt.py
Downloading Qt.py-1.1.0-py2.py3-none-any.whl
Installing collected packages: Qt.py
Successfully installed Qt.py
Cleaning up...

NO packages were installed.
```

Reproduce the Windows command `rez-pip --install Qt.py` using the versions and output shown in the issue, then trace how rez-pip reports installed packages after pip finishes. Done means the Qt.py installation is recognized rather than reporting that no packages were installed.

Written by the indexing model from the issue text.