rez-pip PyQt5 on windows fails to add necessary paths to comands()

Unsure where this breaks, or who to send this too, but running:

`rez-pip --python-version 3.7 -i PyQt5`

Results in
```
qt.qpa.plugin: Could not find the Qt platform plugin "windows" in ""
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.
```

unless the following two paths are added to the commands() of `PyQt5_Qt5` package

```
env.PATH.append('{root}/python/PyQt5/Qt5/bin')
env.QT_PLUGIN_PATH.set('{root}/python/PyQt5/Qt5/plugins')
```

Cheers!

Start by reproducing the Windows failure with `rez-pip --python-version 3.7 -i PyQt5`, then inspect the `PyQt5_Qt5` package's `commands()` configuration. Verify that the required Qt paths are configured there and that the command runs without the Qt platform plugin error.

Written by the indexing model from the issue text.