Use Go1.11 module (go-gitea#5743)

* Migrate to go modules

* make vendor

* Update mvdan.cc/xurls

* make vendor

* Update code.gitea.io/git

* make fmt-check

* Update github.com/go-sql-driver/mysql

* make vendor

CONTRIBUTING.md

@@ -88,7 +88,7 @@ update these instructions.
 ## Vendoring
 
 We keep a cached copy of dependencies within the `vendor/` directory,
-managing updates via [dep](https://github.com/golang/dep).
+managing updates via [Modules](https://golang.org/cmd/go/#hdr-Module_maintenance).
 
 Pull requests should only include `vendor/` updates if they are part of
 the same change, be it a bugfix or a feature addition.
@@ -97,7 +97,7 @@ The `vendor/` update needs to be justified as part of the PR description,
 and must be verified by the reviewers and/or merger to always reference
 an existing upstream commit.
 
-You can find more information on how to get started with it on the [dep project website](https://golang.github.io/dep/docs/introduction.html).
+You can find more information on how to get started with it on the [Modules Wiki](https://github.com/golang/go/wiki/Modules).
 
 ## Translation