plaidbench can't find onnx-plaidml

[znmeb]

How to reproduce (Antergos / Arch Linux):

1. Create a virtual environment and activate it.
2. Install plaidml-keras, plaidbench, and onnx-plaidml in the virtual environment.
3. Run plaidml-setup.
4. plaidbench --blanket-run onnx.

plaidbench --blanket-run onnx
Error: Missing needed packages for benchmark; to fix, pip install onnx-plaidml