vendor: only vendor on emitted binaries

Moves the vendor/ directory to cmd/vendor. Vendored binaries are built
from cmd/, which is backed by symlinks pointing back to repo root.

cmd/vendor/github.com/xiang90/probing/server.go

@@ -0,0 +1,25 @@
+package probing
+
+import (
+	"encoding/json"
+	"net/http"
+	"time"
+)
+
+func NewHandler() http.Handler {
+	return &httpHealth{}
+}
+
+type httpHealth struct {
+}
+
+type Health struct {
+	OK  bool
+	Now time.Time
+}
+
+func (h *httpHealth) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	health := Health{OK: true, Now: time.Now()}
+	e := json.NewEncoder(w)
+	e.Encode(health)
+}