I am still not sure if we want to use Composer to manage the
dependences of r2db, which is only flexihash. It might be easier to
simply add flexihash to the repository of r2db, which we never have
updated in 7+ years. That way we would not need to include the vendor/
folder into the Noblogs production enviroment.

added a theme and a patch as an example

The patch is taken from the git repo noblogs-wp.
It does not apply cleany (yet), since there is an annoying issue with mu-plugins.
They have to be single files, but they are not distributed as single files.
There is a tool which auto-loads them, but I think the mismatch in the directories still causes the patch do not apply nicely.

We have few mu-plugins and we do not update them often, we could think of some solution specific to this problem later.

This is an attempt to re-structre the Wordpress codebase for Noblogs, unsing Composer.