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

* Move migrtions into services and base into modules/migration

* Fix imports

* Fix lint

modules/migration/downloader.go

@@ -0,0 +1,41 @@
+// Copyright 2019 The Gitea Authors. All rights reserved.
+// Copyright 2018 Jonas Franz. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package migration
+
+import (
+	"context"
+
+	"code.gitea.io/gitea/modules/structs"
+)
+
+// GetCommentOptions represents an options for get comment
+type GetCommentOptions struct {
+	Context  IssueContext
+	Page     int
+	PageSize int
+}
+
+// Downloader downloads the site repo information
+type Downloader interface {
+	SetContext(context.Context)
+	GetRepoInfo() (*Repository, error)
+	GetTopics() ([]string, error)
+	GetMilestones() ([]*Milestone, error)
+	GetReleases() ([]*Release, error)
+	GetLabels() ([]*Label, error)
+	GetIssues(page, perPage int) ([]*Issue, bool, error)
+	GetComments(opts GetCommentOptions) ([]*Comment, bool, error)
+	SupportGetRepoComments() bool
+	GetPullRequests(page, perPage int) ([]*PullRequest, bool, error)
+	GetReviews(pullRequestContext IssueContext) ([]*Review, error)
+	FormatCloneURL(opts MigrateOptions, remoteAddr string) (string, error)
+}
+
+// DownloaderFactory defines an interface to match a downloader implementation and create a downloader
+type DownloaderFactory interface {
+	New(ctx context.Context, opts MigrateOptions) (Downloader, error)
+	GitServiceType() structs.GitServiceType
+}