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/EmptyVar/foo/foo.go

@@ -0,0 +1,11 @@
+package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	fmt.Println(injectedMessage())
+}
+
+var myFakeSet struct{}