Introducing Namespace Layer for Overlay Packages

[tianze0926]

Could we introduce a namespace layer to the overlay, allowing us to access packages via pkgs.berberman.fcitx5-pinyin-zhwiki instead of pkgs.fcitx5-pinyin-zhwiki?

This approach would prevent custom packages from conflicting with others that share the same names.

For compatibility reasons, we can create a new overlay rather than modifying the original one.