cache: move unrelated helpers out of CloudCache class (#5439)

* cache: move transfer to objects

Moving it as is for now.

* cache: move link() to checkout

tests/func/test_add.py

@@ -937,8 +937,8 @@ def test_add_file_in_symlink_dir(make_tmp_dir, tmp_dir, dvc, external):
 def test_add_with_cache_link_error(tmp_dir, dvc, mocker, caplog):
     tmp_dir.gen("foo", "foo")
 
-    mocker.patch.object(
-        dvc.cache.local, "_do_link", side_effect=DvcException("link failed")
+    mocker.patch(
+        "dvc.checkout._do_link", side_effect=DvcException("link failed")
     )
     with caplog.at_level(logging.WARNING, logger="dvc"):
         dvc.add("foo")

tests/func/test_s3.py

@@ -55,6 +55,8 @@ def test_copy_singlepart_preserve_etag():
     [S3Tree.PATH_CLS("s3://bucket/"), S3Tree.PATH_CLS("s3://bucket/ns/")],
 )
 def test_link_created_on_non_nested_path(base_info, tmp_dir, dvc, scm):
+    from dvc.checkout import _link
+
     tree = S3Tree(dvc, {"url": str(base_info.parent)})
     cache = CloudCache(tree)
     s3 = cache.tree.s3.meta.client
@@ -62,7 +64,7 @@ def test_link_created_on_non_nested_path(base_info, tmp_dir, dvc, scm):
     s3.put_object(
         Bucket=base_info.bucket, Key=(base_info / "from").path, Body="data"
     )
-    cache.link(base_info / "from", base_info / "to")
+    _link(cache, base_info / "from", base_info / "to")
 
     assert cache.tree.exists(base_info / "from")
     assert cache.tree.exists(base_info / "to")