dvc.fs: move fs to dvc.objects.fs

Some of the APIs/FileSystems are reimported inside dvc.fs module.

tests/func/test_fs.py

@@ -5,9 +5,8 @@ from os.path import join
 
 import pytest
 
-from dvc.fs import get_cloud_fs
+from dvc.fs import LocalFileSystem, get_cloud_fs
 from dvc.fs._callback import DEFAULT_CALLBACK, FsspecCallback
-from dvc.fs.local import LocalFileSystem
 from dvc.repo import Repo