Add git.repo.__all__ and make submodules explicit

- No need to import Repo "as Repo". Some tools only recognize this
  to give the name conceptually public status when it appears in
  type stub files (.pyi files), and listing it in the newly created
  __all__ is sufficient to let humans and all tools know it has
  that status.

- As very recently done in git.refs, this explicitly imports the
  submodules, so it is clear they are available and don't have to
  be explicitly imported. (Fundamental to the way they are used is
  that they will end up being imported in order to import Repo.)

  However, also as in git.refs, it may be that the problems this
  could cause in some inherently flawed but plausible uses are too
  greater for it to be justified. So this may be revisited.

Author: EliahKagan

Diff for: git/repo/__init__.py

@@ -3,4 +3,7 @@
 
 """Initialize the repo package."""
 
-from .base import Repo as Repo  # noqa: F401
+__all__ = ["base", "fun", "Repo"]
+
+from . import base, fun
+from .base import Repo