Move convert package to services (#22264)

Addition to #22256

The `convert` package relies heavily on different models which is
[disallowed by our definition of
modules](https://github.com/go-gitea/gitea/blob/main/CONTRIBUTING.md#design-guideline).
This helps to prevent possible import cycles.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

services/convert/pull_test.go

@@ -0,0 +1,48 @@
+// Copyright 2020 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package convert
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/db"
+	issues_model "code.gitea.io/gitea/models/issues"
+	"code.gitea.io/gitea/models/perm"
+	repo_model "code.gitea.io/gitea/models/repo"
+	"code.gitea.io/gitea/models/unittest"
+	"code.gitea.io/gitea/modules/git"
+	"code.gitea.io/gitea/modules/structs"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestPullRequest_APIFormat(t *testing.T) {
+	// with HeadRepo
+	assert.NoError(t, unittest.PrepareTestDatabase())
+	headRepo := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 1})
+	pr := unittest.AssertExistsAndLoadBean(t, &issues_model.PullRequest{ID: 1})
+	assert.NoError(t, pr.LoadAttributes(db.DefaultContext))
+	assert.NoError(t, pr.LoadIssue(db.DefaultContext))
+	apiPullRequest := ToAPIPullRequest(git.DefaultContext, pr, nil)
+	assert.NotNil(t, apiPullRequest)
+	assert.EqualValues(t, &structs.PRBranchInfo{
+		Name:       "branch1",
+		Ref:        "refs/pull/2/head",
+		Sha:        "4a357436d925b5c974181ff12a994538ddc5a269",
+		RepoID:     1,
+		Repository: ToRepo(db.DefaultContext, headRepo, perm.AccessModeRead),
+	}, apiPullRequest.Head)
+
+	// withOut HeadRepo
+	pr = unittest.AssertExistsAndLoadBean(t, &issues_model.PullRequest{ID: 1})
+	assert.NoError(t, pr.LoadIssue(db.DefaultContext))
+	assert.NoError(t, pr.LoadAttributes(db.DefaultContext))
+	// simulate fork deletion
+	pr.HeadRepo = nil
+	pr.HeadRepoID = 100000
+	apiPullRequest = ToAPIPullRequest(git.DefaultContext, pr, nil)
+	assert.NotNil(t, apiPullRequest)
+	assert.Nil(t, apiPullRequest.Head.Repository)
+	assert.EqualValues(t, -1, apiPullRequest.Head.RepoID)
+}