Update TS linting to import root level interfaces

[web-padawan]

We had some files incorrectly referencing each other which caused errors like this:

      "node_modules/@vaadin/vaadin-app-layout/vaadin-app-layout.d.ts(2,15): error TS2307: Cannot find module './@types/interfaces' or its corresponding type declarations.\n",

This should never happen again, as we have TS in place.

Action point: update packages/**/test/typings to import from the root level entrypoints, not from src.