Make label templates have consistent behavior and priority (#23749)

Fix https://github.com/go-gitea/gitea/issues/23715

Other related PRs:

* #23717
* #23716
* #23719

This PR is different from others, it tries to resolve the problem fundamentally (and brings more benefits)

Although it looks like some more lines are added, actually many new lines are for tests.

----

Before, the code was just "guessing" the file type and try to parse them.

<details>

![image](https://user-images.githubusercontent.com/2114189/228002245-57d58e27-1078-4da9-bf42-5bc0b264c6ce.png)

</details>

This PR:

* Always remember the original option file names, and always use correct parser for them.

* Another benefit is that we can sort the Label Templates now (before there was a map, its key order is undefined)

![image](https://user-images.githubusercontent.com/2114189/228002432-931b9f18-3908-484b-a36b-04760c9ad132.png)

modules/repository/init_test.go

@@ -0,0 +1,30 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package repository
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMergeCustomLabels(t *testing.T) {
+	files := mergeCustomLabelFiles(optionFileList{
+		all:    []string{"a", "a.yaml", "a.yml"},
+		custom: nil,
+	})
+	assert.EqualValues(t, []string{"a.yaml"}, files, "yaml file should win")
+
+	files = mergeCustomLabelFiles(optionFileList{
+		all:    []string{"a", "a.yaml"},
+		custom: []string{"a"},
+	})
+	assert.EqualValues(t, []string{"a"}, files, "custom file should win")
+
+	files = mergeCustomLabelFiles(optionFileList{
+		all:    []string{"a", "a.yml", "a.yaml"},
+		custom: []string{"a", "a.yml"},
+	})
+	assert.EqualValues(t, []string{"a.yml"}, files, "custom yml file should win if no yaml")
+}