Fix offchain created from offchain (#5051)

* refactor: Refactor SubgraphInstance::new_host

* refactor: Move `Hosts` to a module of `instance`

* refactor: Use `mod.rs` style for context and instance

* refactor: Make `mod instance` private to `context`

* fix(offchain): Separate onchain and offchain hosts

The hosts Vec expected the hosts to be pushed in ascending order
of block number. Creating a file ds B within another file ds A violated
that assumption, because B will have the same creation block as A. This
is by design, so the cleanest way to fix this seemed to be separating
the data structures for onchain and offchain hosts. Offchain data sources
do not need to maintain insertion order, as their processing order
ultimately does not impact determinism because they each have their own
causality region. If in the future we introduce a feature that allows
two different offchain data sources to have the same causality region, we maybe
need to revisit this.

* fix: Fix an optimize `*Host::contains`, rename iter_by_address