dvc: separate data and objects

Initial simple separation to get the process of extracting dvc-objects
and dvc-data packages out of dvc.

dvc-objects is for:
 * defining base Object (aka HashFile) and ObjectDB
 * providing tools to work with those objects (check/transfer/etc)
 * etc

dvc-data is for:
 * defining higher level objects (e.g. Tree)
 * bulding objects from the workspace (aka staging)
 * projecting objects back into the workspace (aka checkout)
 * comparing Tree objects (aka diff)
 * defining misc object databases (e.g LocalObjectDB)
 * higher-level transfer (e.g. with Tree indexes)
 * etc

For example, the objects we use in run-cache are based on dvc-objects
and not on dvc-data, as they have absolutely nothing to do with data
management.

There are still some mixups in the code that will be addressed in the
followups.

tests/func/test_external_repo.py

@@ -3,9 +3,9 @@ from unittest.mock import ANY, patch
 
 from scmrepo.git import Git
 
+from dvc.data.stage import stage
+from dvc.data.transfer import transfer
 from dvc.external_repo import CLONES, external_repo
-from dvc.objects.stage import stage
-from dvc.objects.transfer import transfer
 from dvc.utils import relpath
 from dvc.utils.fs import makedirs, remove
 from tests.unit.fs.test_repo import make_subrepo