added refresh db capability and did some refactoring

internal/cache/refresh.go

@@ -0,0 +1,27 @@
+package cache
+
+import "path/filepath"
+
+func (c *Cache) Refresh() error {
+	if !c.mu.TryLock() {
+		return nil
+	}
+	defer c.mu.Unlock()
+
+	for _, repo := range c.mirroredRepos {
+		if err := c.refreshDB(repo); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (c *Cache) refreshDB(repo string) error {
+	dbFile := repo + ".db.tar.gz"
+	dbPath := filepath.Join(repo, "os/x86_64", dbFile)
+	err := c.Fetch(dbPath)
+	if err != nil {
+		return err
+	}
+	return nil
+}