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/.gitignore

@@ -0,0 +1,4 @@
+/vendor
+/bug_test.go
+/coverage.txt
+/.idea