repo: rename odb -> cache

We used to have it named `cache` before, but these days it is clear that
you can have three odbs: just odb (e.g. in-memory), cache (typically local)
or remote (typically cloud). Fixing naming here before proceeding with
introducing `out.odb` and `out.cache`.

In preparation for migration to https://github.com/iterative/dvc-data/pull/304

tests/func/plots/test_show.py

@@ -161,7 +161,7 @@ def test_plots_show_overlap(tmp_dir, dvc, run_copy_metrics, clear_before_run):
     # so as it works even for optimized cases
     if clear_before_run:
         remove(data_dir)
-        remove(dvc.odb.local.path)
+        remove(dvc.cache.local.path)
 
     dvc._reset()