Find yarn dependency package paths via brute force

[jonabc]

Closes #243

After the linked bug had another hit as the underlying issue in #324, I took another look and couldn't find any logical way that made sense to detect yarn package paths under node_modules based on the output from any yarn CLI.

As an alternative, here's a solution that finds and parses every package.json file under node_modules to make a map of all "name@version" => directory pairs. The map is cached so it's at least only evaluated once.

The performance impact isn't too noticeable for small to mid-size projects, but it gets a little more expensive for larger projects. Still, it seems to only take a few seconds for larger projects so it's not too bad.

[jonabc]

/cc @gareth @markphelps 👋 what do you think about this as a solution to #243?

[jonabc]

Moving forward with this solution for now - the yarn source won't work well without this sort of change and the performance hit isn't too bad except for projects with a very large number of dependencies.