modules/programs/bash: move prompt plugins into separate modules

nixos/modules/programs/bash/bash-completion.nix

@@ -0,0 +1,37 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  enable = config.programs.bash.enableCompletion;
+in
+{
+  options = {
+    programs.bash.enableCompletion = mkEnableOption "Bash completion for all interactive bash shells" // {
+      default = true;
+    };
+  };
+
+  config = mkIf enable {
+    programs.bash.promptPluginInit = ''
+      # Check whether we're running a version of Bash that has support for
+      # programmable completion. If we do, enable all modules installed in
+      # the system and user profile in obsolete /etc/bash_completion.d/
+      # directories. Bash loads completions in all
+      # $XDG_DATA_DIRS/bash-completion/completions/
+      # on demand, so they do not need to be sourced here.
+      if shopt -q progcomp &>/dev/null; then
+        . "${pkgs.bash-completion}/etc/profile.d/bash_completion.sh"
+        nullglobStatus=$(shopt -p nullglob)
+        shopt -s nullglob
+        for p in $NIX_PROFILES; do
+          for m in "$p/etc/bash_completion.d/"*; do
+            . $m
+          done
+        done
+        eval "$nullglobStatus"
+        unset nullglobStatus p m
+      fi
+    '';
+  };
+}