cli: refactor into a module

By splitting dvc.cli into a module, we'll be able to share cli-related
utilities, split parser-related stuffs into smaller parts, improve
reusability and improve API for command extensions in the future.

tests/func/test_move.py

@@ -3,9 +3,9 @@ import textwrap
 
 import pytest
 
+from dvc.cli import main
 from dvc.dvcfile import DVC_FILE_SUFFIX
 from dvc.exceptions import DvcException, MoveNotDataSourceError
-from dvc.main import main
 from dvc.utils.serialize import load_yaml
 from tests.basic_env import TestDvc, TestDvcGit
 from tests.func.test_repro import TestRepro