Expose local path of a Library that's remote via appcast 
I have a remote Library with an appcast URL that I've added to sketch. I'd like to get the local file path (not the appcast URL).

Right now getDocument().path returns the appcast URL. My current work around is:

```
library.sketchObject.locationOnDisk().path()
```

Thanks to @mathieudutour for the help via Twitter! https://twitter.com/miked1ck/status/1178768379106058241