plugins/none-ls: refactor using mkSourcePlugin

Introduce `_mk-source-plugin.nix`, which returns a module handling a
specific none-ls source plugin.

This wasn't possible previously due to IFD conflicting with evaluating
module imports.

Adjusted the test to use the `options` provided via module args, and
cleaned up some stuff allowing "unpackaged" sources to not be listed
again in the test file.

plugins/none-ls/_mk-source-plugin.nix

@@ -0,0 +1,58 @@
+# mkSourcePlugin, returns a module
+sourceType: sourceName:
+{
+  config,
+  pkgs,
+  lib,
+  helpers,
+  ...
+}:
+let
+  inherit (import ./packages.nix pkgs) packaged unpackaged;
+
+  cfg = config.plugins.none-ls;
+  cfg' = config.plugins.none-ls.sources.${sourceType}.${sourceName};
+in
+{
+  options.plugins.none-ls.sources.${sourceType}.${sourceName} =
+    {
+      enable = lib.mkEnableOption "the ${sourceName} ${sourceType} source for none-ls";
+      withArgs = helpers.mkNullOrOption helpers.nixvimTypes.strLua ''
+        Raw Lua code passed as an argument to the source's `with` method.
+      '';
+    }
+    # Only declare a package option if a package is required
+    // lib.optionalAttrs (packaged ? ${sourceName} || lib.elem sourceName unpackaged) {
+      package =
+        let
+          pkg = packaged.${sourceName} or null;
+        in
+        lib.mkOption (
+          {
+            type = lib.types.nullOr lib.types.package;
+            description =
+              "Package to use for ${sourceName}."
+              + (lib.optionalString (pkg == null) (
+                "\n\n"
+                + ''
+                  Currently not packaged in nixpkgs.
+                  Either set this to `null` and install ${sourceName} outside of nix,
+                  or set this to a custom nix package.
+                ''
+              ));
+          }
+          // lib.optionalAttrs (pkg != null) { default = pkg; }
+        );
+    };
+
+  config = lib.mkIf (cfg.enable && cfg'.enable) {
+    plugins.none-ls.settings.sources = lib.mkDefault [
+      (
+        "require('null-ls').builtins.${sourceType}.${sourceName}"
+        + lib.optionalString (cfg'.withArgs != null) ".with(${cfg'.withArgs})"
+      )
+    ];
+
+    extraPackages = [ (cfg'.package or null) ];
+  };
+}