bpo-45514: Deprecate importlib resources legacy functions. (GH-29036)

* bpo-45514: Apply changes from importlib_resources@a3ef4128c6

* Mark legacy functions as deprecated in the docs and link to the migration docs in importlib_resources docs.

* Apply changes from importlib_resources@329ae9d5f2c.

* Indicate importlib.resources as a module.

Co-authored-by: Filipe Laíns <lains@riseup.net>