Declare indexer storage backends in the swh.indexer.storage.classes entry point

Normalize the indexer storage db for swh.core 3.6 with improved `swh db` handling
capabilities.

Remove storage/test_init.py, it's now outdated.

Warning: this rename the config entry for indexer storages from
'indexer_storage' to 'indexer.storage'. Bw compat core handle this when
loading the configuration for indexer cli tools and the api server only;
generic loading mechanism will not handle the old name properly
(i.e. when managing the storage db using `swh db` commands).