Hide del util from type checkers

Even though current type checkers don't seem to need it.

As noted in dffa930, it appears neither mypy nor pyright currently
needs `del util` to be guarded by `if not TYPE_CHECKING:` -- they
currently treat util as bound even without it, even with `del util`
present.

It is not obvious that this is the best type checker behavior or
that type checkers will continue to behave this way. (In addition,
it may help humans for all statements whose effects are intended
not to be considered for purposes of static typing to be guarded by
`if not TYPE_CHECKING:`.) So this guards the deletion of util the
same as the binding of __getattr__.

This also moves, clarifies, and expands the commented explanations
of what is going on.

git/__init__.py

@@ -169,6 +169,8 @@
         IndexEntry,
         IndexFile,
         StageType,
+        # NOTE: This tells type checkers what util resolves to. We delete it, and it is
+        # really resolved by __getattr__, which warns. See below on what to use instead.
         util,
     )
     from git.util import (  # @NoMove
@@ -183,27 +185,6 @@
     raise ImportError("%s: %s" % (_exc.__class__.__name__, _exc)) from _exc
 
 
-# NOTE: The expression `git.util` evaluates to git.index.util and `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 *is* what you want, use or import from that, to avoid confusion.
-#
-# - To use the "real" git.util module, write `from git.util import ...`, or if necessary
-#   access it as `sys.modules["git.util"]`.
-#
-# (This differs from other indirect-submodule imports that are unambiguously non-public
-# and subject to immediate removal. Here, the public git.util module, though different,
-# makes less discoverable that the expression `git.util` refers to a non-public
-# attribute of the git module.)
-#
-# This had come about by a wildcard import. Now that all intended imports are explicit,
-# the intuitive but potentially incompatible binding occurs due to the usual rules for
-# Python submodule bindings. So for now we delete that and let __getattr__ handle it.
-#
-del util
-
-
 def _warned_import(message: str, fullname: str) -> "ModuleType":
     import importlib
 
@@ -242,7 +223,36 @@ def _getattr(name: str) -> Any:
     raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
 
 
-if not TYPE_CHECKING:  # Preserve static checking for undefined/misspelled attributes.
+if not TYPE_CHECKING:
+    # NOTE: The expression `git.util` gives git.index.util and `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 *is* what you want, use (or import from) that, to avoid
+    #   confusion.
+    #
+    # - To use the "real" git.util module, write `from git.util import ...`, or if
+    #   necessary access it as `sys.modules["git.util"]`.
+    #
+    # Note also that `import git.util` technically imports the "real" git.util... but
+    # the *expression* `git.util` after doing so is still git.index.util!
+    #
+    # (This situation differs from that of other indirect-submodule imports that are
+    # unambiguously non-public and subject to immediate removal. Here, the public
+    # git.util module, though different, makes less discoverable that the expression
+    # `git.util` refers to a non-public attribute of the git module.)
+    #
+    # This had originally come about by a wildcard import. Now that all intended imports
+    # are explicit, the intuitive but potentially incompatible binding occurs due to the
+    # usual rules for Python submodule bindings. So for now we replace that binding with
+    # git.index.util, delete that, and let __getattr__ handle it and issue a warning.
+    #
+    # For the same runtime behavior, it would be enough to forgo importing util, and
+    # delete util as created naturally; __getattr__ would behave the same. But type
+    # checkers would not know what util refers to when accessed as an attribute of git.
+    del util
+
+    # This is "hidden" to preserve static checking for undefined/misspelled attributes.
     __getattr__ = _getattr
 
 # { Initialize git executable path