Add the nonpublic indirect submodule aliases back for now

These should definitely never be used by code inside or outside of
GitPython, as they have never been public, having even been omitted
by the dynamic (and expansive) technique used to build git.__all__
in the past (which omitted modules intentionally).

However, to ease compatibility, for now they are back. This is so
that the change of making all imports explicit rather than using
wildcards does not break anything. However, code using these names
could never be relied on to work, and these should be considered
eligible for removal, at least from the perspective of code outside
GitPython.

That is for the indirect submodules whose absence as a same-named
direct submodule or attribute listed in __all__ was readily
discoverable.

The more difficult case is util. git.util is a module, git/util.py,
which is how it is treated when it appears immediately after the
"from" keyword, or as a key in sys.modules. However, the expression
`git.util`, and a `from git import util` import, instead access the
separate git.index.util module, which due to a wildcard import has
been an attribute of the top-level git module for a long time.

It may not be safe to change this, because although any code
anywhere is better off not relying on this, this situation hasn't
been (and isn't) immediately clear. To help with it somewhat, this
also includes a detailed comment over where util is imported from
git.index, explaining the situation.

git/__init__.py

@@ -144,6 +144,11 @@
         SymbolicReference,
         Tag,
         TagReference,
+        head,  # noqa: F401  # Nonpublic. May disappear! Use git.refs.head.
+        log,  # noqa: F401  # Nonpublic. May disappear! Use git.refs.log.
+        reference,  # noqa: F401  # Nonpublic. May disappear! Use git.refs.reference.
+        symbolic,  # noqa: F401  # Nonpublic. May disappear! Use git.refs.symbolic.
+        tag,  # noqa: F401  # Nonpublic. May disappear! Use git.refs.tag.
     )
     from git.diff import (  # @NoMove
         INDEX,
@@ -164,7 +169,21 @@
         IndexEntry,
         IndexFile,
         StageType,
-        util,  # noqa: F401  # For backward compatibility.
+        base,  # noqa: F401  # Nonpublic. May disappear! Use git.index.base.
+        fun,  # noqa: F401  # Nonpublic. May disappear! Use git.index.fun.
+        typ,  # noqa: F401  # Nonpublic. May disappear! Use git.index.typ.
+        #
+        # NOTE: The expression `git.util` evaluates to git.index.util, and the import
+        # `from git import util` imports git.index.util, NOT git.util. It may not be
+        # feasible to change this until the next major version, to avoid breaking code
+        # inadvertently relying on it. If git.index.util really is what you want, use or
+        # import from that name, to avoid confusion. To use the "real" git.util module,
+        # write `from git.util import ...`, or access it as `sys.modules["git.util"]`.
+        # (This differs from other historical indirect-submodule imports that are
+        # unambiguously nonpublic and are subject to immediate removal. Here, the public
+        # git.util module, even though different, makes it less discoverable that the
+        # expression `git.util` refers to a non-public attribute of the git module.)
+        util,  # noqa: F401
     )
     from git.util import (  # @NoMove
         Actor,