Fix #9412 - import Sass dependencies in vendor folder

Previous PR: https://github.com/zurb/foundation-sites/pull/9449

Changes:
- Revert https://github.com/zurb/foundation-sites/pull/9430 - Make
`normalize-scss` a dev dependency.
- Copy Sass dependencies to `/_vendor/scss` and import them in Sass
from there.

Why not importing the Sass dependencies as real dependencies and from
`node_modules`: we cannot assume the path to our real dependencies. Old
npm versions store them in our `node_modules` folder, while newer
versions store them in the `node_modules` folder of the root project.