discovery: add mdns plugin

  The initial implementation of mDNS plugin.

Change-Id: I7465b65c2bd90cc1c6400d2345aae2fee0c38363
diff --git a/runtime/internal/discovery/discovery.go b/runtime/internal/discovery/discovery.go
index 79329ec..b502095 100644
--- a/runtime/internal/discovery/discovery.go
+++ b/runtime/internal/discovery/discovery.go
@@ -10,6 +10,8 @@
 	"v.io/v23/discovery"
 )
 
+const pkgPath = "v.io/x/ref/runtime/internal/discovery"
+
 // ds is an implementation of discovery.T.
 type ds struct {
 	plugins []Plugin