The module tries to explain itself as follows:

A keychain here is a set of application-defined indexes for a miniscript descriptor where we can derive script pubkeys at a particular derivation index.

Which is very difficult to read. More importantly it only has one thing in it KeyChainTxOutIndex. We can just have keychain_txout_index.rs. We could move indexers into their own module so indexers/keychain_txout.rs, indexers/spk_txout.rs.