Fixed the breakage for prezto introduced in #1778.
The previous method created issues where certain configuration files would get
replaced by prezto's variants instead of being merged as before. This led to
issues like no config being loaded if `home.zsh.dotDir` was set.
The old method of loading these files has been restored. This fixes the issue.
* zsh: update prezto path structure
The path structure was changed in Nixpkgs and this commit updates
the module to match.
Fixes#1773
* zsh-prezto: fix tests, small tidyup
Co-authored-by: Nick Hu <me@nickhu.co.uk>
