repo: move data_index to .dvc/tmp/index

dvc/repo/__init__.py

@@ -379,23 +379,16 @@ class Repo:
 
     @property
     def data_index(self) -> Optional["DataIndex"]:
-        from appdirs import user_cache_dir
-        from fsspec.utils import tokenize
-
         from dvc_data.index import DataIndex
 
         if not self.config["feature"].get("data_index_cache"):
             return None
 
+        if not self.index_db_dir:
+            return None
+
         if self._data_index is None:
-            cache_dir = user_cache_dir(self.config.APPNAME, self.config.APPAUTHOR)
-            index_dir = os.path.join(
-                cache_dir,
-                "index",
-                "data",
-                # scm.root_dir and repo.root_dir don't match for subrepos
-                tokenize((self.scm.root_dir, self.root_dir)),
-            )
+            index_dir = os.path.join(self.index_db_dir, "data")
             os.makedirs(index_dir, exist_ok=True)
 
             self._data_index = DataIndex.open(os.path.join(index_dir, "db.db"))