This commit replaces the use of `reentry`, for entry point loading,
with `importlib_metadata` and, in turn,
removes the requirement for users to run `reentry scan` after installations.

aiida-core makes heavy use of entry-points to define plugins.
The `reentry` package was introduced to load these plugins since,
at the time, the de facto `pkg_resources` method for using entry points
was too slow, in particular for responsive CLI usage.
This, however, came with the drawback that users must perform an extra step
to register the plugins before aiida-core can be used, or when new plugins are installed.

In recent years `importlib.metadata` and its backport `importlib_metadata`
has replaced `pkg_resources`, and as of python/importlib_metadata#317
is now on a par with `reentry` for performance.

For now, we use `importlib_metadata` for all python versions,
rather than the built-in (as of python 3.8) `importlib.metadata`,
so that we can use the new python 3.10 API and performance boosts.