Client vs Host architeture

Currently, `dplug:host` depends on every plugin client subpackage.

It should be the reverse way, like for `dplug:client`: every format depends on `dplug:host` (which should be runtimefree) and implement one client host.

This will tie client and host implementations for the same format, but I feel like they it's just a bit of code vs ressources. This will allow to create a host program specifically for format X and Y.

Start by mapping the dependency graph between dplug:host, dplug:client, and each format package. Check how host and client implementations are currently connected, then verify that dplug:host can run without plugin clients while each format supplies its own client host. No files or tests are named, so identifying those entry points is part of the work.

Written by the indexing model from the issue text.