Location of x-pack directory should not differ between dev and distributions

[epixa]

The x-pack directory exists in the root of the project during development, but when you run the build task to create artifacts, it moves to node_modules/x-pack. This means there is no reliable way to consistency import oss plugin and core code statically inside x-pack.

It's location should remain static just like our other source code.

I think one of the reasons this was done in the past was to deduplicate shared dependencies, but this should be handled effectively enough now with yarn workspaces.

[elasticmachine]

Pinging @elastic/kibana-operations

[spalger]

The other reason we did that was to enforce that nobody was doing relative imports to core, but plans change

[tylersmalley]

I have put up a draft PR here: #32722