sstables_manager now depends on system_keyspace for access to the
system.sstables table, needed by object storage. This violates
modularity, since sstables_manager is a relatively low-level leaf
module while system_keyspace integrates large parts of the system
(including, indirectly, sstables_manager).
One area where this is grating is sstables::test_env, which has
to include the much higher level cql_test_env to accommodate it.
Fix this by having sstables_manager expose its dependency on
system_keyspace as an interface, sstables_registry, and have
system_keyspace implement the glue logic in
system_keyspace_sstables_manager.
Closesscylladb/scylladb#17868
e48eb76f61