index: fix fs cache location

Existing logic might result in weird fs cache location like
`.dvc/cache/files/md5/fs`, while we really only want
`.dvc/cache/fs`, since it is not an odb cache.

dvc/cachemgr.py

@@ -36,6 +36,7 @@ def _get_odb(
 class CacheManager:
     CACHE_DIR = "cache"
     FILES_DIR = "files"
+    FS_DIR = "fs"
 
     def __init__(self, repo):
         self._repo = repo
@@ -73,6 +74,21 @@ class CacheManager:
         legacy_odb = _get_odb(repo, settings, hash_name="md5-dos2unix", **kwargs)
         self._odb["legacy"] = legacy_odb
 
+    @property
+    def fs_cache(self):
+        """Filesystem-based cache.
+
+        Currently used as a temporary location to download files that we don't
+        yet have a regular oid (e.g. md5) for.
+        """
+        from dvc_data.index import FileStorage
+
+        return FileStorage(
+            key=(),
+            fs=self.local.fs,
+            path=self.local.fs.path.join(self.default_local_cache_dir, self.FS_DIR),
+        )
+
     def _init_odb(self, schemes):
         for scheme in schemes:
             remote = self.config.get(scheme)

dvc/repo/index.py

@@ -186,12 +186,13 @@ def _load_storage_from_out(storage_map, key, out):
             from fsspec.utils import tokenize
 
             # partial import
+            fs_cache = out.repo.cache.fs_cache
             storage_map.add_cache(
                 FileStorage(
                     key,
-                    out.cache.fs,
-                    out.cache.fs.path.join(
-                        out.cache.path, "fs", dep.fs.protocol, tokenize(dep.fs_path)
+                    fs_cache.fs,
+                    fs_cache.fs.path.join(
+                        fs_cache.path, dep.fs.protocol, tokenize(dep.fs_path)
                     ),
                 )
             )