repo: introduce data_index

Extracting from `index.data` as a "root" data index that contains indexes
for different revisions/workspaces.

This is a setup for putting cloud versioning remote indexes and also replacing
old state.

tests/func/test_data_cloud.py

@@ -199,7 +199,8 @@ def test_verify_hashes(tmp_dir, scm, dvc, mocker, tmp_path_factory, local_remote
     hash_spy = mocker.spy(dvc_data.hashfile.hash, "file_md5")
 
     dvc.pull()
-    assert hash_spy.call_count == 0
+    # NOTE: 1 is for index.data_tree building
+    assert hash_spy.call_count == 1
 
     # Removing cache will invalidate existing state entries
     dvc.cache.local.clear()
@@ -207,7 +208,7 @@ def test_verify_hashes(tmp_dir, scm, dvc, mocker, tmp_path_factory, local_remote
     dvc.config["remote"]["upstream"]["verify"] = True
 
     dvc.pull()
-    assert hash_spy.call_count == 4
+    assert hash_spy.call_count == 6
 
 
 @flaky(max_runs=3, min_passes=1)