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/Struct/foo/goose.go

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