Move migrations into services and base into modules/migration (#17663)

* Move migrtions into services and base into modules/migration

* Fix imports

* Fix lint

services/migrations/git.go

@@ -0,0 +1,51 @@
+// Copyright 2019 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package migrations
+
+import (
+	"context"
+
+	base "code.gitea.io/gitea/modules/migration"
+)
+
+var (
+	_ base.Downloader = &PlainGitDownloader{}
+)
+
+// PlainGitDownloader implements a Downloader interface to clone git from a http/https URL
+type PlainGitDownloader struct {
+	base.NullDownloader
+	ownerName string
+	repoName  string
+	remoteURL string
+}
+
+// NewPlainGitDownloader creates a git Downloader
+func NewPlainGitDownloader(ownerName, repoName, remoteURL string) *PlainGitDownloader {
+	return &PlainGitDownloader{
+		ownerName: ownerName,
+		repoName:  repoName,
+		remoteURL: remoteURL,
+	}
+}
+
+// SetContext set context
+func (g *PlainGitDownloader) SetContext(ctx context.Context) {
+}
+
+// GetRepoInfo returns a repository information
+func (g *PlainGitDownloader) GetRepoInfo() (*base.Repository, error) {
+	// convert github repo to stand Repo
+	return &base.Repository{
+		Owner:    g.ownerName,
+		Name:     g.repoName,
+		CloneURL: g.remoteURL,
+	}, nil
+}
+
+// GetTopics return empty string slice
+func (g PlainGitDownloader) GetTopics() ([]string, error) {
+	return []string{}, nil
+}