Public Ocluster_worker library

[art-w]

For current-bench and sandmark, we need to experiment with a different kind of worker that build images, and then run them with custom settings to ensure that the benchmarks are stable. This PR exposes the internal library Ocluster_worker to help us extend the existing build behaviour.

However there is an issue with the obuilder git submodule that causes linker errors when trying to use the resulting ocluster-worker library: It gets compiled with the pinned obuilder, but it still requires a dependency on the publicly released obuilder (causing a conflict, see ocaml/dune#3911). That's why the second commit removes the submodule and replaces it with an opam pin. Is there an alternative solution to this issue that you would prefer?

[art-w]

I've rebased the PR as current-bench still depends on this to implement its custom benchmarking workers: they are roughly a simplified version of the standard worker, but we need to enable some runtime settings to stabilize the benchmarks (which isolated cpu cores to use, a tmpfs, etc)

I respent a few hours trying not to remove the obuilder git submodule, because I understand it is very convenient... but nothing worked within dune. Some alternatives that would work though:

• Functorizing the worker with obuilder signature, such that we can choose a non-vendored version later, but it's clearly super invasive.
• Inserting a remove in the *.opam files to get rid of the vendored version when people install it:

build: [
  ["dune" "subst"] {dev}
  ["rm" "-Rf" "obuilder"]       <-----
  ["dune" "build" "-p" ...]
]

(I don't know how to keep the dune-project generation of the opam files with the latest)
Let me know if you have some other ideas or preferences :)

[MisterDA]

Circling back on this after a long while, as I'm going to cut a release of ocluster soon.
Do you really need to track the development branch of ocluster, either by pinning or sub-moduling? If not, what if I simply expose the library as part of ocluster, or even through a new package if needed, and you simply depend on ocluster.0.2.1 and link with the library? When installed through opam, ocluster will depend on the opam package, and the submodule won't be pulled.
cc @ElectreAAS