fix: keep both cases on lakka and retropie

Both run on Linux but inherited RetroArch's case folding, so a core
file differing from a platform name only by case left their packs:
MesenCE's bios.col and np2kai's lowercase 2608_*.wav, which those
cores open with that exact case. They are now case-sensitive, the
shared groups fold only where the platform does, and Lakka gets its
own pack instead of sharing RetroArch's.

tests/test_verify_pack_choice.py

@@ -39,7 +39,7 @@ class VerifyPackChoice(unittest.TestCase):
         self.db = self.out / "db.json"
         self.db.write_text(json.dumps({"files": {}}))
         self.stem = generate_pack._platform_pack_stem(
-            ["retroarch", "lakka"], "retroarch", "platforms"
+            ["retroarch"], "retroarch", "platforms"
         )
 
     def _args(self, **overrides) -> argparse.Namespace: