fetchNpmDeps: allow package-json.lock symlinks, update hint

      buildPhase = ''

        runHook preBuild

        if [[ ! -f package-lock.json ]]; then

        if [[ ! -e package-lock.json ]]; then

          echo

          echo "ERROR: The package-lock.json file does not exist!"

          echo

          echo "package-lock.json is required to make sure that npmDepsHash doesn't change"

          echo "when packages are updated on npm."

          echo

          echo "Hint: You can use the patches attribute to add a package-lock.json manually to the build."

          echo "Hint: You can copy a vendored package-lock.json file via postPatch."

          echo

          exit 1