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/null_downloader.go

@@ -0,0 +1,87 @@
+// Copyright 2021 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 migration
+
+import (
+	"context"
+	"net/url"
+)
+
+// NullDownloader implements a blank downloader
+type NullDownloader struct {
+}
+
+var (
+	_ Downloader = &NullDownloader{}
+)
+
+// SetContext set context
+func (n NullDownloader) SetContext(_ context.Context) {}
+
+// GetRepoInfo returns a repository information
+func (n NullDownloader) GetRepoInfo() (*Repository, error) {
+	return nil, &ErrNotSupported{Entity: "RepoInfo"}
+}
+
+// GetTopics return repository topics
+func (n NullDownloader) GetTopics() ([]string, error) {
+	return nil, &ErrNotSupported{Entity: "Topics"}
+}
+
+// GetMilestones returns milestones
+func (n NullDownloader) GetMilestones() ([]*Milestone, error) {
+	return nil, &ErrNotSupported{Entity: "Milestones"}
+}
+
+// GetReleases returns releases
+func (n NullDownloader) GetReleases() ([]*Release, error) {
+	return nil, &ErrNotSupported{Entity: "Releases"}
+}
+
+// GetLabels returns labels
+func (n NullDownloader) GetLabels() ([]*Label, error) {
+	return nil, &ErrNotSupported{Entity: "Labels"}
+}
+
+// GetIssues returns issues according start and limit
+func (n NullDownloader) GetIssues(page, perPage int) ([]*Issue, bool, error) {
+	return nil, false, &ErrNotSupported{Entity: "Issues"}
+}
+
+// GetComments returns comments according the options
+func (n NullDownloader) GetComments(GetCommentOptions) ([]*Comment, bool, error) {
+	return nil, false, &ErrNotSupported{Entity: "Comments"}
+}
+
+// GetPullRequests returns pull requests according page and perPage
+func (n NullDownloader) GetPullRequests(page, perPage int) ([]*PullRequest, bool, error) {
+	return nil, false, &ErrNotSupported{Entity: "PullRequests"}
+}
+
+// GetReviews returns pull requests review
+func (n NullDownloader) GetReviews(pullRequestContext IssueContext) ([]*Review, error) {
+	return nil, &ErrNotSupported{Entity: "Reviews"}
+}
+
+// FormatCloneURL add authentification into remote URLs
+func (n NullDownloader) FormatCloneURL(opts MigrateOptions, remoteAddr string) (string, error) {
+	if len(opts.AuthToken) > 0 || len(opts.AuthUsername) > 0 {
+		u, err := url.Parse(remoteAddr)
+		if err != nil {
+			return "", err
+		}
+		u.User = url.UserPassword(opts.AuthUsername, opts.AuthPassword)
+		if len(opts.AuthToken) > 0 {
+			u.User = url.UserPassword("oauth2", opts.AuthToken)
+		}
+		return u.String(), nil
+	}
+	return remoteAddr, nil
+}
+
+// SupportGetRepoComments return true if it supports get repo comments
+func (n NullDownloader) SupportGetRepoComments() bool {
+	return false
+}