fix: the created fetcher should have type {tarball: FetchFunction}

shrinkwrap.yaml

@@ -1,5 +1,6 @@
 dependencies:
   '@pnpm/fetcher-base': 1.0.0
+  '@pnpm/tarball-fetcher': 'link:../__package_previews__/tarball-fetcher/@pnpm/tarball-fetcher'
   '@types/mem': 1.1.2
   '@types/mz': 0.0.32
   '@types/node': 9.6.2
@@ -1876,7 +1877,7 @@ packages:
     resolution:
       integrity: sha1-5a2ryKz0CPY4X8dklWhMiOavaJo=
 registry: 'https://registry.npmjs.org/'
-shrinkwrapMinorVersion: 4
+shrinkwrapMinorVersion: 5
 shrinkwrapVersion: 3
 specifiers:
   '@pnpm/fetcher-base': ^1.0.0

src/index.ts

@@ -1,5 +1,6 @@
 import logger from '@pnpm/logger'
 import {
+  FetchFunction,
   FetchOptions,
   FetchResult,
 } from '@pnpm/fetcher-base'
@@ -35,17 +36,7 @@ export default function (
     ignoreFile?: IgnoreFunction,
     offline?: boolean,
   },
-): {
-  tarball: (
-    resolution: {
-      integrity?: string,
-      registry?: string,
-      tarball: string,
-    },
-    target: string,
-    opts: FetchOptions,
-  ) => Promise<{}>
-} {
+): { tarball: FetchFunction } {
   const download = createDownloader({
     alwaysAuth: opts.alwaysAuth || false,
     registry: opts.registry,