Option to provide absolute path to pyls

Hi,
Since virtual-environments are very commonly used, its a bit cumbersome to keep installing `pyls` in each environment.
Is there a way to set an absolute path to the `pyls` ( or perhaps I missed this configuration ? )

for eg.
```
➯ ➯ ➯ which pyls
~/anaconda3/bin/pyls
```
vs
```
➯ ➯ ➯ which python
~/anaconda3/envs/my-env/bin/python
```

I'd like to set `pyls` to point to 1st one.

Start by locating the configuration or entry point that selects the pyls executable; the issue does not name a file or test. Verify how the existing Python environment is chosen, then confirm that users can provide an absolute pyls path independent of the active virtual environment.

Rédigé par le modèle d'indexation à partir du texte de l'issue.