Reduce a bit of duplication by lifting out a `get_config()`
helper function to a new `internal` package.

Removes some duplication between `startup.py` and `reload.py` with
respect to processing the config for `ep_types`.

This centralizes the handling of importing the correct `importlib`
metadata, and the logic for finding the entry points for a specific
entrypoint group name.

Quoting the upstream docs:
> Changed in version 3.12: The “selectable” entry points were introduced
> in importlib_metadata 3.6 and Python 3.10. Prior to those changes,
> entry_points accepted no parameters and always returned a dictionary of
> entry points, keyed by group. With importlib_metadata 5.0 and Python
> 3.12, entry_points always returns an EntryPoints object.

Rather than try to gate the updated logic on both the `importlib`
version and the Python version we instead update `get_entry_points()` to
check whether it has a `dict` or not after calling `entry_points()` and
act accordingly.