Fix creation of zip packages in Windows (#1490)

Package Registry was using the OS-native relative paths directly when
building zip packages. So on windows this was using back slashes, but
names in zip headers are expected to use forward slashes.

Tests were passing only because they only check the headers, and the
names there were converted to the expected format when reading the zip.

Author: jsoriano

archiver/archive.go

@@ -93,7 +93,7 @@ func buildArchiveHeader(info os.FileInfo, relativePath string) (*zip.FileHeader,
 	}
 
 	header.Method = zip.Deflate
-	header.Name = relativePath
+	header.Name = filepath.ToSlash(relativePath)
 	if info.IsDir() && !strings.HasSuffix(header.Name, "/") {
 		header.Name = header.Name + "/"
 	}

main_test.go

@@ -689,10 +689,7 @@ func listArchivedFiles(t *testing.T, body []byte) []byte {
 	var listing bytes.Buffer
 
 	for _, f := range zipReader.File {
-		// f.Name is populated from the zip file directly and is not validated for correctness.
-		// Using filepath.ToSlash(f.Name) ensures that the file name has the expected format
-		// regardless of the OS.
-		listing.WriteString(fmt.Sprintf("%d %s\n", f.UncompressedSize64, filepath.ToSlash(f.Name)))
+		listing.WriteString(fmt.Sprintf("%d %s\n", f.UncompressedSize64, f.Name))
 	}
 	return listing.Bytes()
 }