add go-plugin deps to vendor

vendor/github.com/hashicorp/go-plugin/process.go

@@ -0,0 +1,24 @@
+package plugin
+
+import (
+	"time"
+)
+
+// pidAlive checks whether a pid is alive.
+func pidAlive(pid int) bool {
+	return _pidAlive(pid)
+}
+
+// pidWait blocks for a process to exit.
+func pidWait(pid int) error {
+	ticker := time.NewTicker(1 * time.Second)
+	defer ticker.Stop()
+
+	for range ticker.C {
+		if !pidAlive(pid) {
+			break
+		}
+	}
+
+	return nil
+}