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!"