Same motivation as in https://github.com/NixOS/nixpkgs/pull/28378.
zsh.initExtra parameter can be used by external modules which can
redefine user aliases. This change will give user-defined aliases
the highest priority.
1. It slows down the initial start: it takes around 2s at first launch,
and around 0.25s for the following launches;
2. It seems to be redundant since just installing zsh package gives
working completions with correct $fpath set.
