repo: move state/index/data_index to site_cache_dir

We use all of these for optimization and should not depend on a potentially slow or broken
filesystem on which repository resides (could be HDD or CIFS/NFS/overlayfs/etc).

This PR means a few things:
    * all odd filesystems that dvc repo might be on will work quickly out of the box
    * when we start caching cloned repositories for `dvc get/import`, we will be
      able to cache indexes for them as well (note that bare clones don't have a
      worktree so there is no place for .dvc/tmp with the old approach).
    * `index.dir` and `state.dir` are now obsolete.
    * `dvc init` will cleanup site_cache_dir.

Related #5562

tests/func/test_state.py

@@ -1,7 +1,5 @@
 import os
-import re
 
-from dvc.repo import Repo
 from dvc_data.hashfile.hash import file_md5
 from dvc_data.hashfile.hash_info import HashInfo
 from dvc_data.hashfile.state import State
@@ -71,15 +69,3 @@ def test_get_unused_links(tmp_dir, dvc):
             dvc.fs,
         )
     ) == {"bar"}
-
-
-def test_state_dir_config(make_tmp_dir, dvc):
-    assert dvc.state.tmp_dir == dvc.tmp_dir
-
-    index_dir = str(make_tmp_dir("tmp_index"))
-    repo = Repo(config={"state": {"dir": index_dir}})
-    assert os.path.dirname(repo.state.tmp_dir) == os.path.join(index_dir, ".dvc")
-    assert re.match(
-        r"^test_state_dir_config0-([0-9a-f]+)$",
-        os.path.basename(repo.state.tmp_dir),
-    )

tests/unit/remote/test_remote.py

@@ -1,11 +1,8 @@
-import os
-
 import pytest
 from dvc_gs import GSFileSystem
 from dvc_s3 import S3FileSystem
 
 from dvc.fs import get_cloud_fs
-from dvc_data.hashfile.db import get_index
 
 
 def test_remote_with_hash_jobs(dvc):
@@ -57,18 +54,3 @@ def test_makedirs_not_create_for_top_level_path(fs_cls, dvc, mocker):
 
     fs.makedirs(url)
     assert not mocked_client.called
-
-
-def test_remote_index_dir_config(make_tmp_dir, dvc):
-    index_dir = str(make_tmp_dir("tmp_index"))
-    with dvc.config.edit() as conf:
-        conf["index"]["dir"] = index_dir
-        conf["remote"]["s3"] = {"url": "s3://bucket/name"}
-
-    dvc.root_dir = "/usr/local/test_repo"
-    dvc.dvc_dir = "/usr/local/test_repo/.dvc"
-    dvc.__dict__.pop("local_dvc_dir")
-
-    assert os.path.dirname(
-        get_index(dvc.cloud.get_remote_odb(name="s3")).index_dir
-    ) == os.path.join(index_dir, ".dvc", "test_repo-a473718", "index")