Add a demo program to demonstrate the fix for cross-package interface private method calls. This demo uses go/types.Scope.Insert which has private methods in the go/types package, making it a good test case for the bug fix in PR #1371. Generated with [codeagent](https://github.com/qbox/codeagent) Co-authored-by: luoliwoshang <luoliwoshang@users.noreply.github.com>