prevent duplicate plugin discovery on misconfigured pythons

for example, `venv`-virtualenvs on fedora have both `lib` and `lib64` on
`sys.path` despite them being the same.  this causes
`importlib.metadata.distributions` to double-discover.

```console
$ docker run --rm -t fedora:latest bash -c 'dnf install -qq -y python3 >& /dev/null && python3 -m venv venv && venv/bin/pip -qq install cfgv && venv/bin/python - <<< "from importlib.metadata import distributions; print(len([d for d in distributions() if d.name == '"'"'cfgv'"'"']))"'
2
```