Convert already_processed to a set()

We basically removed the recursive functionality
of already_processed in this PR:
https://github.com/MrBin99/django-vite/pull/109/files
We did that because the list concatenation was getting
so large for some users. But we can keep the benefits
of saving the recursive tags we've already_processed
(and do so without excessive bloat) by turning
already_processed into a set().

django_vite/core/asset_loader.py

@@ -1,6 +1,6 @@
 import json
 from pathlib import Path
-from typing import Dict, List, Callable, NamedTuple, Optional, Union
+from typing import Dict, List, Callable, NamedTuple, Optional, Union, Set
 from urllib.parse import urljoin
 import warnings
 
@@ -427,13 +427,13 @@ def _load_css_files_of_asset(
     class GeneratedCssFilesOutput(NamedTuple):
         # list of generated CSS tags
         tags: List[Tag]
-        # list of already processed CSS tags
-        already_processed: List[str]
+        # set of already processed CSS tags
+        already_processed: Set[str]
 
     def _generate_css_files_of_asset(
         self,
         path: str,
-        already_processed: Optional[List[str]] = None,
+        already_processed: Optional[Set[str]] = None,
         tag_generator: Callable[[str], Tag] = TagGenerator.stylesheet,
     ) -> GeneratedCssFilesOutput:
         """
@@ -449,21 +449,22 @@ def _generate_css_files_of_asset(
             already_processed -- List of already processed css paths
         """
         if already_processed is None:
-            already_processed = []
+            already_processed = set()
         tags: List[Tag] = []
         manifest_entry = self.manifest.get(path)
 
         for import_path in manifest_entry.imports:
-            new_tags, _ = self._generate_css_files_of_asset(
+            new_tags, new_already_processed = self._generate_css_files_of_asset(
                 import_path, already_processed, tag_generator
             )
+            already_processed.update(new_already_processed)
             tags.extend(new_tags)
 
         for css_path in manifest_entry.css:
             if css_path not in already_processed:
                 url = self._get_production_server_url(css_path)
                 tags.append(tag_generator(url))
-                already_processed.append(css_path)
+                already_processed.add(css_path)
 
         return self.GeneratedCssFilesOutput(tags, already_processed)