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/extensions/README.md

@@ -0,0 +1,5 @@
+# Extensions
+
+This directory contains support code for building Gnostic extensions and associated examples.
+
+Extensions are used to compile vendor or specification extensions into protocol buffer structures.