osbuild-composer: Rename module to github.com/osbuild/osbuild-composer

This should be the best practice according to other popular go projects:
- https://github.com/prometheus/prometheus
- https://github.com/syncthing/syncthing
- https://github.com/drone/drone
- https://github.com/hashicorp/terraform

Also, this change fixes go get command (it currently fails due to bad package
name).

go.mod

@@ -1,4 +1,4 @@
-module osbuild-composer
+module github.com/osbuild/osbuild-composer
 
 go 1.12