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/google.golang.org/grpc/install_gae.sh

@@ -0,0 +1,6 @@
+#!/bin/bash
+
+TMP=$(mktemp -d /tmp/sdk.XXX) \
+&& curl -o $TMP.zip "https://storage.googleapis.com/appengine-sdks/featured/go_appengine_sdk_linux_amd64-1.9.68.zip" \
+&& unzip -q $TMP.zip -d $TMP \
+&& export PATH="$PATH:$TMP/go_appengine"