nix flake refactor: avoid with lib; & use lib.optional*

Upstream Nixpkgs has been pushing the style of removing ``with lib;``
with the reasoning of clarity around where variables in scope come that
hurt readability & static analysis[*].

While on the topic of ``lib``, introduce ``lib.optional`` &
``lib.optionalString`` in a few more places to tidy up the code while
using common library patterns.

.. [*] https://nix.dev/guides/best-practices#with-scopes

Format: text/x-rst

flake.nix

@@ -10,32 +10,34 @@
     in
     {
       nixosModule = { config, ... }:
-        with nixpkgs.lib;
         let
+          inherit (nixpkgs) lib;
           cfg = config.networking.stevenBlackHosts;
-          alternatesList = (if cfg.blockFakenews then [ "fakenews" ] else []) ++
-                           (if cfg.blockGambling then [ "gambling" ] else []) ++
-                           (if cfg.blockPorn then [ "porn" ] else []) ++
-                           (if cfg.blockSocial then [ "social" ] else []);
+          alternatesList =
+            (lib.optional cfg.blockFakenews "fakenews")
+            ++ (lib.optional cfg.blockGambling "gambling")
+            ++ (lib.optional cfg.blockPorn "porn")
+            ++ (lib.optional cfg.blockSocial "social");
           alternatesPath = "alternates/" + builtins.concatStringsSep "-" alternatesList + "/";
         in
         {
           options.networking.stevenBlackHosts = {
-            enable = mkEnableOption "Steven Black's hosts file";
-            enableIPv6 = mkEnableOption "IPv6 rules" // {
+            enable = lib.mkEnableOption "Steven Black's hosts file";
+            enableIPv6 = lib.mkEnableOption "IPv6 rules" // {
               default = config.networking.enableIPv6;
             };
-            blockFakenews = mkEnableOption "fakenews hosts entries";
-            blockGambling = mkEnableOption "gambling hosts entries";
-            blockPorn = mkEnableOption "porn hosts entries";
-            blockSocial = mkEnableOption "social hosts entries";
+            blockFakenews = lib.mkEnableOption "fakenews hosts entries";
+            blockGambling = lib.mkEnableOption "gambling hosts entries";
+            blockPorn = lib.mkEnableOption "porn hosts entries";
+            blockSocial = lib.mkEnableOption "social hosts entries";
           };
-          config = mkIf cfg.enable {
+          config = lib.mkIf cfg.enable {
             networking.extraHosts =
               let
-                orig = builtins.readFile ("${self}/" + (if alternatesList != [] then alternatesPath else "") + "hosts");
+                orig = builtins.readFile ("${self}/" + (lib.optionalString (alternatesList != []) alternatesPath) + "hosts");
                 ipv6 = builtins.replaceStrings [ "0.0.0.0" ] [ "::" ] orig;
-              in orig + (optionalString cfg.enableIPv6 ("\n" + ipv6));
+              in
+              orig + (lib.optionalString cfg.enableIPv6 ("\n" + ipv6));
           };
         };