Add __all__ and imports in git.objects.submodule

This is the top-level of the git.objects.submodule subpackage, for
its own Python submodules.

This appears only not to have been done before because of a cyclic
import problem involving imports that are no longer present.

Author: EliahKagan

git/objects/__init__.py

@@ -26,7 +26,6 @@
 from .base import IndexObject, Object
 from .blob import Blob
 from .commit import Commit
-from .submodule.base import Submodule, UpdateProgress
-from .submodule.root import RootModule, RootUpdateProgress
+from .submodule import RootModule, RootUpdateProgress, Submodule, UpdateProgress
 from .tag import TagObject
 from .tree import Tree, TreeModifier

git/objects/submodule/__init__.py

@@ -1,5 +1,16 @@
 # This module is part of GitPython and is released under the
 # 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
 
-# NOTE: Cannot import anything here as the top-level __init__ has to handle
-# our dependencies.
+__all__ = [
+    "base",
+    "root",
+    "util",
+    "Submodule",
+    "UpdateProgress",
+    "RootModule",
+    "RootUpdateProgress",
+]
+
+from . import base, root, util
+from .base import Submodule, UpdateProgress
+from .root import RootModule, RootUpdateProgress