Compatibility considerations:
- importlib.readers and importlib.simple are public, so probably need aliases.
- Same abc classes.
Should we embark on an effort to migrate users to the new names in `importlib.resources`, or simply leave the `importlib.{mod}` names as preferred? My instinct is to do the former, mainly because then the names would be congruent between `importlib.resources` and `importlib_resources`. But I'm also reluctant to introduce a deprecation (even deferred) comparable to what happened with `collections.abc`.