Forcing fpath to contain unique values increases startup speed by
eliminating extra work of processing duplicated folders.
In addition, it increases startup time when zsh is enabled in both
system and home configuration due to having the same fpath value
between different compinit calls.
Fixes https://github.com/rycee/home-manager/issues/108.
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.
