repo-prune: group packages with signatures

Ever since the download scripts copies over the original mtime from
CI to the package, the mtimes of the package file and the signature
files diverge. Since the prune script only goes by mtime this means
packages get removed before signatures and which leads to some stray
signature files on the server from time to time.

Only delete a file if all existing related files should also be deleted.

Fixes #8

msys2-repo-prune

@@ -166,6 +166,24 @@ def get_files_to_prune(target_dir, time_delta):
         for group in group_by_package_name_and_ext(entries):
             paths_to_delete.update(group_get_paths_too_old(group, not_to_delete))
 
+    def get_related_files(path: str) -> list[str]:
+        """Returns a list of related files which should be deleted together with the given file"""
+
+        if not path.endswith(".sig"):
+            return [path + ".sig"]
+        else:
+            return [path[:-4]]
+
+    def related_also_to_delete(path: str) -> bool:
+        """Returns True if all related files are also to be deleted, or don't exist"""
+
+        for related in get_related_files(path):
+            if related not in paths_to_delete and os.path.exists(related):
+                return False
+        return True
+
+    paths_to_delete = set(p for p in paths_to_delete if related_also_to_delete(p))
+
     return paths_to_delete