replace "dep" with "go mod"

A simple "go mod init && go mod tidy && go mod vendor" was enough and
mostly seems to have picked the same revisions of the dependencies as
before. Notable exceptions are klog and gogo/protobuf.

vendor/github.com/json-iterator/go/build.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+set -e
+set -x
+
+if [ ! -d /tmp/build-golang/src/github.com/json-iterator ]; then
+    mkdir -p /tmp/build-golang/src/github.com/json-iterator
+    ln -s $PWD /tmp/build-golang/src/github.com/json-iterator/go
+fi
+export GOPATH=/tmp/build-golang
+go get -u github.com/golang/dep/cmd/dep
+cd /tmp/build-golang/src/github.com/json-iterator/go
+exec $GOPATH/bin/dep ensure -update