s3: call s3.ListFiles only on publish path in LinkFromPool

instead of caching the whole s3 bucket, cache only the pool path. this
requires an additional parameter, and since this is an interface, all
implementations need to follow. might help in other backends too.

closes #1181

deb/package.go

@@ -621,9 +621,7 @@ func (p *Package) LinkFromPool(publishedStorage aptly.PublishedStorage, packageP
 			return err
 		}
 
-		publishedDirectory := filepath.Join(prefix, relPath)
-
-		err = publishedStorage.LinkFromPool(publishedDirectory, f.Filename, packagePool, sourcePoolPath, f.Checksums, force)
+		err = publishedStorage.LinkFromPool(prefix, relPath, f.Filename, packagePool, sourcePoolPath, f.Checksums, force)
 		if err != nil {
 			return err
 		}