Accept imports from the ckeditor5 package from `src/` directory only

[pomek]

📝 Provide detailed reproduction steps (if any)

See: 0b9e98f

import 'ckeditor5/packages/ckeditor5-ui/theme/components/responsive-form/responsiveform.css';

The snippet above uses the wrong path. Even if it works on our local machines, it will not work when integrators create a custom build, and all packages will be installed from npm.

[pomek]

ESlint should prevent writing such paths.