support multi-module (#4)

* add multimodule
* re-package modules

util/fs.go

@@ -0,0 +1,19 @@
+package util
+
+import (
+	"os"
+	"testing"
+)
+
+func CreateWorkDir(t *testing.T) (string, func()) {
+	t.Helper()
+	dir, err := os.MkdirTemp("", "fs-test-workdir")
+	if err != nil {
+		t.Fatalf("create temp dir: %s", err)
+	}
+	return dir, func() {
+		if err := os.RemoveAll(dir); err != nil {
+			t.Fatalf("remove temp dir: %s", err)
+		}
+	}
+}