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/NiladicIdentity/foo/foo.go

@@ -6,8 +6,6 @@ func main() {
 	fmt.Println(injectedMessage())
 }
 
-//goose:provide
-
 // provideMessage provides a friendly user greeting.
 func provideMessage() string {
 	return "Hello, World!"

internal/goose/testdata/NiladicIdentity/foo/foo_goose.go

@@ -2,6 +2,10 @@
 
 package main
 
-//goose:use provideMessage
+import (
+	"codename/goose"
+)
 
-func injectedMessage() string
+func injectedMessage() string {
+	panic(goose.Use(provideMessage))
+}