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_merge_driver.py

@@ -2,7 +2,7 @@ import os
 
 import pytest
 
-from dvc.main import main
+from dvc.cli import main
 from dvc.utils.fs import remove