Make zshrc more predictable

Originially, `programs.zsh` sets default values for some
initialisation scripts.
Nix resolves the case of multiple values by concatenating them all.
It is however impossible to predict where the default script will be
inserted; but we never want the default value to override the
user-specified ones.
Now, it doesn't set default values; almost everything is hardcoded at
the begining of the file.

nixos/modules/programs/zsh/oh-my-zsh.nix

@@ -42,6 +42,10 @@ in
     };
 
     config = mkIf cfg.enable {
+
+      # Prevent zsh from overwriting oh-my-zsh's prompt
+      programs.zsh.promptInit = mkDefault "";
+
       environment.systemPackages = with pkgs; [ oh-my-zsh ];
 
       programs.zsh.interactiveShellInit = with pkgs; with builtins; ''

nixos/modules/programs/zsh/zsh.nix

@@ -97,45 +97,6 @@ in
 
   config = mkIf cfg.enable {
 
-    programs.zsh = {
-
-      shellInit = ''
-        . ${config.system.build.setEnvironment}
-
-        ${cfge.shellInit}
-      '';
-
-      loginShellInit = cfge.loginShellInit;
-
-      interactiveShellInit = ''
-        # history defaults
-        SAVEHIST=2000
-        HISTSIZE=2000
-        HISTFILE=$HOME/.zsh_history
-
-        setopt HIST_IGNORE_DUPS SHARE_HISTORY HIST_FCNTL_LOCK
-
-        # Tell zsh how to find installed completions
-        for p in ''${(z)NIX_PROFILES}; do
-          fpath+=($p/share/zsh/site-functions $p/share/zsh/$ZSH_VERSION/functions $p/share/zsh/vendor-completions)
-        done
-
-        ${if cfg.enableCompletion then "autoload -U compinit && compinit" else ""}
-
-        ${optionalString (cfg.enableAutosuggestions)
-          "source ${pkgs.zsh-autosuggestions}/share/zsh-autosuggestions/zsh-autosuggestions.zsh"
-        }
-
-        ${zshAliases}
-        ${cfg.promptInit}
-
-        ${cfge.interactiveShellInit}
-
-        HELPDIR="${pkgs.zsh}/share/zsh/$ZSH_VERSION/help"
-      '';
-
-    };
-
     environment.etc."zshenv".text =
       ''
         # /etc/zshenv: DO NOT EDIT -- this file has been generated automatically.
@@ -146,6 +107,10 @@ in
         if [ -n "$__ETC_ZSHENV_SOURCED" ]; then return; fi
         export __ETC_ZSHENV_SOURCED=1
 
+        . ${config.system.build.setEnvironment}
+
+        ${cfge.shellInit}
+
         ${cfg.shellInit}
 
         # Read system-wide modifications.
@@ -163,6 +128,8 @@ in
         if [ -n "$__ETC_ZPROFILE_SOURCED" ]; then return; fi
         __ETC_ZPROFILE_SOURCED=1
 
+        ${cfge.loginShellInit}
+
         ${cfg.loginShellInit}
 
         # Read system-wide modifications.
@@ -182,8 +149,34 @@ in
 
         . /etc/zinputrc
 
+        # history defaults
+        SAVEHIST=2000
+        HISTSIZE=2000
+        HISTFILE=$HOME/.zsh_history
+
+        setopt HIST_IGNORE_DUPS SHARE_HISTORY HIST_FCNTL_LOCK
+
+        HELPDIR="${pkgs.zsh}/share/zsh/$ZSH_VERSION/help"
+
+        ${optionalString cfg.enableCompletion "autoload -U compinit && compinit"}
+
+        ${optionalString (cfg.enableAutosuggestions)
+          "source ${pkgs.zsh-autosuggestions}/share/zsh-autosuggestions/zsh-autosuggestions.zsh"
+        }
+
+        ${zshAliases}
+
+        ${cfge.interactiveShellInit}
+
         ${cfg.interactiveShellInit}
 
+        ${cfg.promptInit}
+
+        # Tell zsh how to find installed completions
+        for p in ''${(z)NIX_PROFILES}; do
+          fpath+=($p/share/zsh/site-functions $p/share/zsh/$ZSH_VERSION/functions $p/share/zsh/vendor-completions)
+        done
+
         # Read system-wide modifications.
         if test -f /etc/zshrc.local; then
           . /etc/zshrc.local