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/golang.org/x/oauth2/.travis.yml

@@ -0,0 +1,13 @@
+language: go
+
+go:
+  - tip
+
+install:
+  - export GOPATH="$HOME/gopath"
+  - mkdir -p "$GOPATH/src/golang.org/x"
+  - mv "$TRAVIS_BUILD_DIR" "$GOPATH/src/golang.org/x/oauth2"
+  - go get -v -t -d golang.org/x/oauth2/...
+
+script:
+  - go test -v golang.org/x/oauth2/...