Use Go1.11 module (#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

vendor/github.com/src-d/gcfg/README

@@ -0,0 +1,4 @@
+Gcfg reads INI-style configuration files into Go structs;
+supports user-defined types and subsections.
+
+Package docs: https://godoc.org/gopkg.in/gcfg.v1