cmd: add import comments

See https://golang.org/cmd/go/#hdr-Import_path_checking

cmd/relaynode/relaynode.go

@@ -6,7 +6,7 @@
 //
 // Deprecated: this program will be soon deleted. The replacement is
 // cmd/tailscaled.
-package main
+package main // import "tailscale.com/cmd/relaynode"
 
 import (
 	"context"

cmd/tailscale/ipn.go

@@ -4,7 +4,7 @@
 
 // The tailscale command is the Tailscale command-line client. It interacts
 // with the tailscaled client daemon.
-package main
+package main // import "tailscale.com/cmd/tailscale"
 
 import (
 	"context"

cmd/tailscaled/ipnd.go

@@ -7,7 +7,7 @@
 //
 // It primarily supports Linux, though other systems will likely be
 // supported in the future.
-package main
+package main // import "tailscale.com/cmd/tailscaled"
 
 import (
 	"context"