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. Doing
it improves consistency, since all the other subpackages under the
top-level git package already do this.

The modattrs.py script reveals the four new attributes of
git.objects.submodule for the four classes obtained by the new
imports, as expected. (The explicit module imports do not change
the attribues because they are the same attributes as come into
existence due to those Python submodules being imported anywhere in
any style.)

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