Fully Support and Test PNPM

[rolandpeelen]

We're already testing for Yarn / Yarn Workspaces. Not Pnpm. This results in some issues that arise in an ad-hoc fashion. Perhaps it's better to get those out of the way from the get-go and use the snapshot tests for PNPM as well. This issue is to track the progress.

• - Insert into tests. Perhaps we can use this test repo from this issue
• - Sourcedirs support
• - Dynamically determine package manager, and add it to build state so we don't have to ad-hoc try and go up the tree to find packages?