Put less emphasis on plugin build isolation, to get a better DevEx out
of it. The crux of the test is the database extraction part, not the
plugin build.
We are no longer bound to the platform-specific directories, so simplify the test organization.
If you don't want this change, just skip merging this PR. It's purely optional.
I kept the platform-specific directories around under `kotlin`,
but you could also easily merge all these together if you find them unhelpful.
I'll leave that change to you.
