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_config.py

@@ -3,8 +3,8 @@ import textwrap
 
 import pytest
 
+from dvc.cli import main
 from dvc.config import Config, ConfigError
-from dvc.main import main
 
 
 def test_config_set(tmp_dir, dvc):