CLI: ensure consistent path handling #1049

The CLI needs a single helper util function which will resolve paths in a consistent way.

When the user says openfn job.js, what is job.js relative to? The current working dir of course. But any path resolution needs to be consistent about that. We also need to support stuff like /job.js and ~./job.js.

It's not super hard to do this, and most of the CLI does it in a consistent way. It's what baseDir is sort of supposed to sort out.

But there isn't a single helper function. Ideally users contributing new commands can just do resolvePath(options, path) and getCommandRoot() or something, and we have a single function which works it all out without needing much thought