Not working with Yarn 3.x

#### Issue description:

Project is using Yarn 3.x, and it's 'pnp' feature, so, it has a yarn.lock file, but, since it's using pnp, there is no node_modules folder, and synp is complaining about that.

Start by reproducing the issue with Yarn 3.x using Plug'n'Play, where yarn.lock exists without node_modules. Inspect how synp locates and reads the Yarn lockfile and relies on node_modules; done when the conversion works in this setup without requiring a node_modules folder.

Written by the indexing model from the issue text.