Draft since currently we can't specify directories with holes in mounts.
Also, it seems there isn't any error checking for file existence yet, which can be dangerous, but only for local development. In production, terser will throw. But if we eventually move with #29280, we need this to throw in case a mount does not exist.
Fixes #27412.
Preview: https://deploy-preview-29283--twbs-bootstrap.netlify.app/