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/gogo/protobuf/proto/extensions.go

@@ -527,6 +527,7 @@ func ExtensionDescs(pb Message) ([]*ExtensionDesc, error) {
 // SetExtension sets the specified extension of pb to the specified value.
 func SetExtension(pb Message, extension *ExtensionDesc, value interface{}) error {
 	if epb, ok := pb.(extensionsBytes); ok {
+		ClearExtension(pb, extension)
 		newb, err := encodeExtension(extension, value)
 		if err != nil {
 			return err