Merge pull request 'buildClan: Add deprecation warning' (#4384) from Qubasa/clan-core:migrate_away_buildClan into main

Reviewed-on: https://git.clan.lol/clan/clan-core/pulls/4384
Reviewed-by: hsjobeki <hsjobeki@gmail.com>

lib/modules/default.nix

@@ -7,8 +7,29 @@
 }:
 rec {
   buildClan =
-    # TODO: Once all templates and docs are migrated add: lib.warn "'buildClan' is deprecated. Use 'clan-core.lib.clan' instead"
-    module: (clan module).config;
+    module:
+    lib.warn ''
+      ==================== DEPRECATION NOTICE ====================
+      Please migrate 
+      from: 'clan = inputs.<clan-core>.lib.buildClan' 
+      to  : 'clan = inputs.<clan-core>.lib.clan'
+      in your flake.nix.
+
+      Please also migrate
+      from: 'inherit (clan) nixosConfigurations clanInternals; '
+      to  : "
+              inherit (clan.config) nixosConfigurations clanInternals;
+              clan = clan.config;
+            "
+      in your flake.nix.
+
+      Reason: 
+      - Improves consistency between flake-parts and non-flake-parts users.
+              
+      - It also allows us to use the top level attribute 'clan' to expose
+        attributes that can be used for cross-clan functionality.
+      ============================================================
+    '' (clan module).config;
 
   clan =
     {