goose: use marker functions instead of comments

To avoid making this CL too large, I did not migrate the existing goose
comments through the repository.  This will be addressed in a subsequent
CL.

Reviewed-by: Tuo Shan <shantuo@google.com>

internal/goose/testdata/Vendor/foo/goose.go

@@ -3,9 +3,10 @@
 package main
 
 import (
-	_ "bar"
+	"bar"
+	"codename/goose"
 )
 
-//goose:use "bar".Message
-
-func injectedMessage() string
+func injectedMessage() string {
+	panic(goose.Use(bar.ProvideMessage))
+}

internal/goose/testdata/Vendor/foo/vendor/bar/bar.go

@@ -1,8 +1,6 @@
 // Package bar is the vendored copy of bar which contains the real provider.
 package bar
 
-//goose:provide Message
-
 // ProvideMessage provides a friendly user greeting.
 func ProvideMessage() string {
 	return "Hello, World!"