Location of built source code in plugins

[kimjoar]

[placeholder — needs more details]

For core plugins and xpack I expect we'll have the source at the root (e.g. in src/), then the built JS in target/something. E.g. in my build exploration PR (#14333) the TypeScript gets built into target/tsc.

However, for pre-built plugins, do we want to require the same?

[epixa]

This is for development, right? In that case, I think we should put them inside a top level dev directory. This is getting close to the problems I wanted to address with a new directory structure overall, as I don't think this sort of question should really be hard to answer: #12466