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/googleapis/gnostic/OpenAPIv2/README.md

@@ -0,0 +1,16 @@
+# OpenAPI v2 Protocol Buffer Models
+
+This directory contains a Protocol Buffer-language model
+and related code for supporting OpenAPI v2.
+
+Gnostic applications and plugins can use OpenAPIv2.proto
+to generate Protocol Buffer support code for their preferred languages.
+
+OpenAPIv2.go is used by Gnostic to read JSON and YAML OpenAPI 
+descriptions into the Protocol Buffer-based datastructures 
+generated from OpenAPIv2.proto.
+
+OpenAPIv2.proto and OpenAPIv2.go are generated by the Gnostic 
+compiler generator, and OpenAPIv2.pb.go is generated by 
+protoc, the Protocol Buffer compiler, and protoc-gen-go, the
+Protocol Buffer Go code generation plugin.