lib.options.mkPackageOption: use lib.showAttrPath

Make use of `lib.showAttrPath` instead of manually doing `concatStringsSep "."`.

This means edge-cases such as the attr-path including names that are not
valid nix identifiers will be handled better.

See:
- https://nix.dev/manual/nix/2.26/language/identifiers
- https://nixos.org/manual/nixpkgs/unstable/#function-library-lib.attrsets.showAttrPath

lib/options.nix

···
   inherit (lib.attrsets)
     attrByPath
     optionalAttrs
+    showAttrPath
     ;
   inherit (lib.strings)
     concatMapStrings
···
     ;
   inherit (lib.lists)
     last
+    toList
     ;
   prioritySuggestion = ''
     Use `lib.mkForce value` or `lib.mkDefault value` to change the priority on any of these definitions.
···
     }:
     let
       name' = if isList name then last name else name;
-      default' = if isList default then default else [ default ];
-      defaultText = concatStringsSep "." default';
+      default' = toList default;
+      defaultText = showAttrPath default';
       defaultValue = attrByPath default' (throw "${defaultText} cannot be found in ${pkgsText}") pkgs;
       defaults =
         if default != null then
           {
             default = defaultValue;
-            defaultText = literalExpression ("${pkgsText}." + defaultText);
+            defaultText = literalExpression "${pkgsText}.${defaultText}";
           }
         else
           optionalAttrs nullable {
···
       }
       // optionalAttrs (example != null) {
         example = literalExpression (
-          if isList example then "${pkgsText}." + concatStringsSep "." example else example
+          if isList example then "${pkgsText}.${showAttrPath example}" else example
         );
       }
     );

lib/tests/modules.sh

···
 checkConfigOutput '^"hello"$' config.nullablePackageWithDefault.pname ./declare-mkPackageOption.nix
 checkConfigOutput '^"myPkgs\.hello"$' options.packageWithPkgsText.defaultText.text ./declare-mkPackageOption.nix
 checkConfigOutput '^"hello-other"$' options.packageFromOtherSet.default.pname ./declare-mkPackageOption.nix
+checkConfigOutput '^"hello"$' config.packageInvalidIdentifier.pname ./declare-mkPackageOption.nix
+checkConfigOutput '^"pkgs\.\\"123\\"\.\\"with\\\\\\"quote\\"\.hello"$' options.packageInvalidIdentifier.defaultText.text ./declare-mkPackageOption.nix
+checkConfigOutput '^"pkgs\.\\"123\\"\.\\"with\\\\\\"quote\\"\.hello"$' options.packageInvalidIdentifierExample.example.text ./declare-mkPackageOption.nix
 
 # submoduleWith
 

lib/tests/modules/declare-mkPackageOption.nix

···
         };
       in
       lib.mkPackageOption myPkgs "hello" { };
+
+    packageInvalidIdentifier =
+      let
+        myPkgs."123"."with\"quote" = { inherit (pkgs) hello; };
+      in
+      lib.mkPackageOption myPkgs [ "123" "with\"quote" "hello" ] { };
+
+    packageInvalidIdentifierExample = lib.mkPackageOption pkgs "hello" {
+      example = [
+        "123"
+        "with\"quote"
+        "hello"
+      ];
+    };
   };
 }