fix: installation of tarball from subfolder

Ref https://github.com/pnpm/supi/issues/10

src/fetch.ts

@@ -127,6 +127,7 @@ export default async function fetch (
         pkg,
         storeIndex: options.storeIndex,
         verifyStoreIntegrity: options.verifyStoreIntegrity,
+        prefix: options.prefix,
       })
     }
 
@@ -156,6 +157,7 @@ function fetchToStore (opts: {
   pkg?: Package,
   storeIndex: Store,
   verifyStoreIntegrity: boolean,
+  prefix: string,
 }): {
   fetchingFiles: Promise<PackageContentInfo>,
   fetchingPkg: Promise<Package>,
@@ -228,6 +230,7 @@ function fetchToStore (opts: {
             pkgId: opts.pkgId,
             storePath: opts.storePath,
             offline: opts.offline,
+            prefix: opts.prefix,
           })
         }(),
         // removing only the folder with the unpacked files

src/fetchResolution.ts

@@ -20,6 +20,7 @@ export type FetchOptions = {
   got: Got,
   storePath: string,
   offline: boolean,
+  prefix: string,
 }
 
 export type PackageDist = {
@@ -79,7 +80,7 @@ function execGit (args: string[], opts?: Object) {
 
 export function fetchFromTarball (dir: string, dist: PackageDist, opts: FetchOptions) {
   if (dist.tarball.startsWith('file:')) {
-    dist = Object.assign({}, dist, {tarball: dist.tarball.slice(5)})
+    dist = Object.assign({}, dist, {tarball: path.join(opts.prefix, dist.tarball.slice(5))})
     return fetchFromLocalTarball(dir, dist)
   } else {
     return fetchFromRemoteTarball(dir, dist, opts)