supporting workspace?

It seems like this crate does not work if it is used in a workspace. The error is:
```
thread 'main' panicked at 'Failed to acquire build-time information: Os { code: 2, kind: NotFound, message: "No such file or directory" }'
```

By a little bit of investigation, I think the culprit is `get_build_deps`, where it errored out when trying to open the lockfile.