prefetch-npm-deps: add support for `npm-shrinkwrap.json`

        buildPhase = ''

          runHook preBuild

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

          if [[ -f npm-shrinkwrap.json ]]; then

            local -r srcLockfile="npm-shrinkwrap.json"

          elif [[ -f package-lock.json ]]; then

            local -r srcLockfile="package-lock.json"

          else

            echo

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

            echo "ERROR: No lock file!"

            echo

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

            echo "when packages are updated on npm."

            echo "package-lock.json or npm-shrinkwrap.json is required to make sure"

            echo "that npmDepsHash doesn't change when packages are updated on npm."

            echo

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

            echo

            exit 1

          fi

          prefetch-npm-deps package-lock.json $out

          prefetch-npm-deps $srcLockfile $out

          runHook postBuild

        '';