all: Become a Go module (fixes #5148) (#5384)

* go mod init; rm -rf vendor

* tweak proto files and generation

* go mod vendor

* clean up build.go

* protobuf literals in tests

* downgrade gogo/protobuf

vendor/github.com/prometheus/procfs/Makefile

@@ -0,0 +1,18 @@
+ci: fmt lint test
+
+fmt:
+	! gofmt -l *.go | read nothing
+	go vet
+
+lint:
+	go get github.com/golang/lint/golint
+	golint *.go
+
+test: sysfs/fixtures/.unpacked
+	go test -v ./...
+
+sysfs/fixtures/.unpacked: sysfs/fixtures.ttar
+	./ttar -C sysfs -x -f sysfs/fixtures.ttar
+	touch $@
+
+.PHONY: fmt lint test ci