flake: provide a single NixOS module

Instead of providing two NixOS modules (one for using system pkgs,
one for using locked pkgs), provide a single module with option
`useVersionLockedPkgs`.

This fixes that all nix-bitcoin options are displayed twice
on search.nixos.org:
https://search.nixos.org/flakes?type=options&query=clightning

examples/flakes/flake.nix

@@ -8,19 +8,11 @@
     nixosConfigurations.mynode = nix-bitcoin.inputs.nixpkgs.lib.nixosSystem {
       system = "x86_64-linux";
       modules = [
-        ## Note:
+        nix-bitcoin.nixosModule
-        ## If you use a custom nixpkgs version for evaluating your system,
-        ## consider using `withLockedPkgs` instead of `withSystemPkgs` to use the exact
-        ## pkgs versions for nix-bitcoin services that are tested by nix-bitcoin.
-        ## The downsides are increased evaluation times and increased system
-        ## closure size.
-        #
-        # nix-bitcoin.nixosModules.withLockedPkgs
-        nix-bitcoin.nixosModules.withSystemPkgs
 
-        ## Optional:
+        # Optional:
-        ## Import the secure-node preset, an opinionated config to enhance security
+        # Import the secure-node preset, an opinionated config to enhance security
-        ## and privacy.
+        # and privacy.
         #
         # "${nix-bitcoin}/modules/presets/secure-node.nix"
 
@@ -46,6 +38,15 @@
             isNormalUser = true;
             password = "a";
           };
+
+          # If you use a custom nixpkgs version for evaluating your system
+          # (instead of `nix-bitcoin.inputs.nixpkgs` like in this example),
+          # consider setting `useVersionLockedPkgs = true` to use the exact pkgs
+          # versions for nix-bitcoin services that are tested by nix-bitcoin.
+          # The downsides are increased evaluation times and increased system
+          # closure size.
+          #
+          # nix-bitcoin.useVersionLockedPkgs = true;
         }
       ];
     };

flake.nix

@@ -28,19 +28,33 @@
         nbPkgs = lib.mkNbPkgs { inherit (final) system; pkgs = final; };
       in removeAttrs nbPkgs [ "pinned" "nixops19_09" "krops" ];
 
-      nixosModules = {
+      nixosModule = { config, pkgs, lib, ... }: {
-        # Uses the default system pkgs for nix-bitcoin.pkgs
-        withSystemPkgs =  { pkgs, ... }: {
         imports = [ ./modules/modules.nix ];
-          nix-bitcoin.pkgs = (lib.mkNbPkgs { inherit (pkgs) system; inherit pkgs; }).modulesPkgs;
+
+        options = with lib; {
+          nix-bitcoin.useVersionLockedPkgs = mkOption {
+            type = types.bool;
+            default = false;
+            description = ''
+              Use the nixpkgs version locked by this flake for `nix-bitcoin.pkgs`.
+              Only relevant if you are using a nixpkgs version for evaluating your system
+              that differs from the one that is locked by this flake (via input `nixpkgs`).
+              If this is the case, enabling this option may result in a more stable system
+              because the nix-bitcoin services use the exact pkgs versions that are tested
+              by nix-bitcoin.
+              The downsides are increased evaluation times and increased system
+              closure size.
+
+              If `false`, the default system pkgs are used.
+            '';
+          };
         };
 
-        # Uses the nixpkgs version locked by this flake for nix-bitcoin.pkgs.
+        config = {
-        # More stable, but slightly slower to evaluate and needs more space if the
+          nix-bitcoin.pkgs =
-        # locked and the system nixpkgs versions differ.
+            if config.nix-bitcoin.useVersionLockedPkgs
-        withLockedPkgs =  { config, ... }: {
+            then (self.lib.mkNbPkgs { inherit (config.nixpkgs) system; }).modulesPkgs
-          imports = [ ./modules/modules.nix ];
+            else (self.lib.mkNbPkgs { inherit (pkgs) system; inherit pkgs; }).modulesPkgs;
-          nix-bitcoin.pkgs = (lib.mkNbPkgs { inherit (config.nixpkgs) system; }).modulesPkgs;
         };
       };
 
@@ -77,7 +91,7 @@
               inherit system;
               configuration = {
                 imports = [
-                  nix-bitcoin.nixosModules.withSystemPkgs
+                  nix-bitcoin.nixosModule
                   "${nix-bitcoin}/modules/presets/secure-node.nix"
                 ];