Cyberstorm API: add CyberstormPackageCategorySerializer

It felt a bit dirty to import such a simple serializer from another
app, especially since that serializer is marked experimental.

Refs TS-1860

django/thunderstore/api/cyberstorm/serializers/community.py

@@ -1,11 +1,7 @@
 from rest_framework import serializers
 
-from thunderstore.community.api.experimental.serializers import (
-    PackageCategoryExperimentalSerializer,
-)
-from thunderstore.community.models import PackageCategory
-from thunderstore.repository.api.experimental.serializers import (
-    CommunityFilteredModelChoiceField,
+from thunderstore.api.cyberstorm.serializers.shared import (
+    CyberstormPackageCategorySerializer,
 )
 
 
@@ -19,7 +15,7 @@ class CyberstormCommunitySerializer(serializers.Serializer):
     icon_url = serializers.CharField(required=False)
     total_download_count = serializers.SerializerMethodField()
     total_package_count = serializers.SerializerMethodField()
-    package_categories = PackageCategoryExperimentalSerializer(many=True)
+    package_categories = CyberstormPackageCategorySerializer(many=True)
 
     def get_total_download_count(self, obj) -> int:
         return obj.aggregated.download_count

django/thunderstore/api/cyberstorm/serializers/shared.py

@@ -0,0 +1,10 @@
+"""
+Do not import/export these through __init__.py to avoid circular imports
+in other serializer files.
+"""
+from rest_framework import serializers
+
+
+class CyberstormPackageCategorySerializer(serializers.Serializer):
+    name = serializers.CharField()
+    slug = serializers.SlugField()