Use absolut jsonnet imports whenever possible

There are still some dependencies that we need to make work to fully
deactivate the legacyImports in the future. I'll start opening PRs
against those other repositories.

jsonnet/kube-prometheus/jsonnetfile.json

@@ -13,7 +13,7 @@
     {
       "source": {
         "git": {
-          "remote": "https://github.com/coreos/etcd",
+          "remote": "https://github.com/etcd-io/etcd",
           "subdir": "Documentation/etcd-mixin"
         }
       },