impermanence: split plaintext store to own file (this will bypass some recursion in the next patch)

modules/impermanence/stores/default.nix

@@ -6,12 +6,10 @@ in
 {
   imports = [
     ./crypt.nix
+    ./plaintext.nix
     ./private.nix
   ];
 
   config = lib.mkIf cfg.enable {
-    sane.impermanence.stores."plaintext" = {
-      mountpt = "/nix/persist";
-    };
   };
 }

modules/impermanence/stores/plaintext.nix

@@ -0,0 +1,9 @@
+{ config, lib, ... }:
+
+let
+  cfg = config.sane.impermanence;
+in lib.mkIf cfg.enable {
+  sane.impermanence.stores."plaintext" = {
+    mountpt = "/nix/persist";
+  };
+}