Move all structs related to Taskfile to its own package

internal/taskfile/taskfile.go

@@ -0,0 +1,26 @@
+package taskfile
+
+// Taskfile represents a Taskfile.yml
+type Taskfile struct {
+	// TODO: version is still not used
+	Version int
+	Tasks   Tasks
+}
+
+// UnmarshalYAML implements yaml.Unmarshaler interface
+func (tf *Taskfile) UnmarshalYAML(unmarshal func(interface{}) error) error {
+	if err := unmarshal(&tf.Tasks); err == nil {
+		return nil
+	}
+
+	var taskfile struct {
+		Version int
+		Tasks   Tasks
+	}
+	if err := unmarshal(&taskfile); err != nil {
+		return err
+	}
+	tf.Version = taskfile.Version
+	tf.Tasks = taskfile.Tasks
+	return nil
+}